Merge branch 'dl/difftool-mergetool'

Update "git difftool" and "git mergetool" so that the combinations
of {diff,merge}.{tool,guitool} configuration variables serve as
fallback settings of each other in a sensible order.

* dl/difftool-mergetool:
  difftool: fallback on merge.guitool
  difftool: make --gui, --tool and --extcmd mutually exclusive
  mergetool: fallback to tool when guitool unavailable
  mergetool--lib: create gui_mode function
  mergetool: use get_merge_tool function
  t7610: add mergetool --gui tests
  t7610: unsuppress output

9 files changed, 180 insertions, 93 deletions

diff --git a/Documentation/git-difftool.txt b/Documentation/git-difftool.txt
index 96c26e6aa8..484c485fd0 100644
--- a/Documentation/git-difftool.txt
+++ b/Documentation/git-difftool.txt
@@ -90,7 +90,9 @@ instead.  `--no-symlinks` is the default on Windows.
 	When 'git-difftool' is invoked with the `-g` or `--gui` option
 	the default diff tool will be read from the configured
 	`diff.guitool` variable instead of `diff.tool`. The `--no-gui`
-	option can be used to override this setting.
+	option can be used to override this setting. If `diff.guitool`
+	is not set, we will fallback in the order of `merge.guitool`,
+	`diff.tool`, `merge.tool` until a tool is found.
 
 --[no-]trust-exit-code::
 	'git-difftool' invokes a diff tool individually on each file.
diff --git a/Documentation/git-mergetool--lib.txt b/Documentation/git-mergetool--lib.txt
index 055550b2bc..4da9d24096 100644
--- a/Documentation/git-mergetool--lib.txt
+++ b/Documentation/git-mergetool--lib.txt
@@ -28,7 +28,9 @@ to define the operation mode for the functions listed below.
 FUNCTIONS
 ---------
 get_merge_tool::
-	returns a merge tool.
+	returns a merge tool. the return code is 1 if we returned a guessed
+	merge tool, else 0. '$GIT_MERGETOOL_GUI' may be set to 'true' to
+	search for the appropriate guitool.
 
 get_merge_tool_cmd::
 	returns the custom command for a merge tool.
diff --git a/Documentation/git-mergetool.txt b/Documentation/git-mergetool.txt
index 0c7975a050..6b14702e78 100644
--- a/Documentation/git-mergetool.txt
+++ b/Documentation/git-mergetool.txt
@@ -83,7 +83,9 @@ success of the resolution after the custom tool has exited.
 --gui::
 	When 'git-mergetool' is invoked with the `-g` or `--gui` option
 	the default merge tool will be read from the configured
-	`merge.guitool` variable instead of `merge.tool`.
+	`merge.guitool` variable instead of `merge.tool`. If
+	`merge.guitool` is not set, we will fallback to the tool
+	configured under `merge.tool`.
 
 --no-gui::
 	This overrides a previous `-g` or `--gui` setting and reads the
diff --git a/builtin/difftool.c b/builtin/difftool.c
index 04ffa1d943..53188df714 100644
--- a/builtin/difftool.c
+++ b/builtin/difftool.c
@@ -24,7 +24,6 @@
 #include "object-store.h"
 #include "dir.h"
 
-static char *diff_gui_tool;
 static int trust_exit_code;
 
 static const char *const builtin_difftool_usage[] = {
@@ -34,11 +33,6 @@ static const char *const builtin_difftool_usage[] = {
 
 static int difftool_config(const char *var, const char *value, void *cb)
 {
-	if (!strcmp(var, "diff.guitool")) {
-		diff_gui_tool = xstrdup(value);
-		return 0;
-	}
-
 	if (!strcmp(var, "difftool.trustexitcode")) {
 		trust_exit_code = git_config_bool(var, value);
 		return 0;
@@ -735,8 +729,11 @@ int cmd_difftool(int argc, const char **argv, const char *prefix)
 		setenv(GIT_WORK_TREE_ENVIRONMENT, absolute_path(get_git_work_tree()), 1);
 	}
 
-	if (use_gui_tool && diff_gui_tool && *diff_gui_tool)
-		setenv("GIT_DIFF_TOOL", diff_gui_tool, 1);
+	if (use_gui_tool + !!difftool_cmd + !!extcmd > 1)
+		die(_("--gui, --tool and --extcmd are mutually exclusive"));
+
+	if (use_gui_tool)
+		setenv("GIT_MERGETOOL_GUI", "true", 1);
 	else if (difftool_cmd) {
 		if (*difftool_cmd)
 			setenv("GIT_DIFF_TOOL", difftool_cmd, 1);
diff --git a/git-difftool--helper.sh b/git-difftool--helper.sh
index 7bfb6737df..46af3e60b7 100755
--- a/git-difftool--helper.sh
+++ b/git-difftool--helper.sh
@@ -71,7 +71,7 @@ then
 	then
 		merge_tool="$GIT_DIFF_TOOL"
 	else
-		merge_tool="$(get_merge_tool)" || exit
+		merge_tool="$(get_merge_tool)"
 	fi
 fi
 
diff --git a/git-mergetool--lib.sh b/git-mergetool--lib.sh
index aaa4eed0bc..204a5acd66 100644
--- a/git-mergetool--lib.sh
+++ b/git-mergetool--lib.sh
@@ -80,14 +80,18 @@ show_tool_names () {
 	}
 }
 
-diff_mode() {
+diff_mode () {
 	test "$TOOL_MODE" = diff
 }
 
-merge_mode() {
+merge_mode () {
 	test "$TOOL_MODE" = merge
 }
 
+gui_mode () {
+	test "$GIT_MERGETOOL_GUI" = true
+}
+
 translate_merge_tool_path () {
 	echo "$1"
 }
@@ -351,20 +355,36 @@ guess_merge_tool () {
 }
 
 get_configured_merge_tool () {
-	# If first argument is true, find the guitool instead
-	if test "$1" = true
-	then
-		gui_prefix=gui
-	fi
-
-	# Diff mode first tries diff.(gui)tool and falls back to merge.(gui)tool.
-	# Merge mode only checks merge.(gui)tool
+	keys=
 	if diff_mode
 	then
-		merge_tool=$(git config diff.${gui_prefix}tool || git config merge.${gui_prefix}tool)
+		if gui_mode
+		then
+			keys="diff.guitool merge.guitool diff.tool merge.tool"
+		else
+			keys="diff.tool merge.tool"
+		fi
 	else
-		merge_tool=$(git config merge.${gui_prefix}tool)
+		if gui_mode
+		then
+			keys="merge.guitool merge.tool"
+		else
+			keys="merge.tool"
+		fi
 	fi
+
+	merge_tool=$(
+		IFS=' '
+		for key in $keys
+		do
+			selected=$(git config $key)
+			if test -n "$selected"
+			then
+				echo "$selected"
+				return
+			fi
+		done)
+
 	if test -n "$merge_tool" && ! valid_tool "$merge_tool"
 	then
 		echo >&2 "git config option $TOOL_MODE.${gui_prefix}tool set to unknown tool: $merge_tool"
@@ -404,14 +424,17 @@ get_merge_tool_path () {
 }
 
 get_merge_tool () {
+	is_guessed=false
 	# Check if a merge tool has been configured
 	merge_tool=$(get_configured_merge_tool)
 	# Try to guess an appropriate merge tool if no tool has been set.
 	if test -z "$merge_tool"
 	then
 		merge_tool=$(guess_merge_tool) || exit
+		is_guessed=true
 	fi
 	echo "$merge_tool"
+	test "$is_guessed" = false
 }
 
 mergetool_find_win32_cmd () {
diff --git a/git-mergetool.sh b/git-mergetool.sh
index 01b9ad59b2..88fa6a914a 100755
--- a/git-mergetool.sh
+++ b/git-mergetool.sh
@@ -389,7 +389,7 @@ print_noop_and_exit () {
 
 main () {
 	prompt=$(git config --bool mergetool.prompt)
-	gui_tool=false
+	GIT_MERGETOOL_GUI=false
 	guessed_merge_tool=false
 	orderfile=
 
@@ -416,10 +416,10 @@ main () {
 			esac
 			;;
 		--no-gui)
-			gui_tool=false
+			GIT_MERGETOOL_GUI=false
 			;;
 		-g|--gui)
-			gui_tool=true
+			GIT_MERGETOOL_GUI=true
 			;;
 		-y|--no-prompt)
 			prompt=false
@@ -449,12 +449,8 @@ main () {
 
 	if test -z "$merge_tool"
 	then
-		# Check if a merge tool has been configured
-		merge_tool=$(get_configured_merge_tool $gui_tool)
-		# Try to guess an appropriate merge tool if no tool has been set.
-		if test -z "$merge_tool"
+		if ! merge_tool=$(get_merge_tool)
 		then
-			merge_tool=$(guess_merge_tool) || exit
 			guessed_merge_tool=true
 		fi
 	fi
diff --git a/t/t7610-mergetool.sh b/t/t7610-mergetool.sh
index a9fb971615..5b61c10a9c 100755
--- a/t/t7610-mergetool.sh
+++ b/t/t7610-mergetool.sh
@@ -130,14 +130,55 @@ test_expect_success 'custom mergetool' '
 	test_when_finished "git reset --hard" &&
 	git checkout -b test$test_count branch1 &&
 	git submodule update -N &&
-	test_must_fail git merge master >/dev/null 2>&1 &&
-	( yes "" | git mergetool both >/dev/null 2>&1 ) &&
+	test_must_fail git merge master &&
+	( yes "" | git mergetool both ) &&
 	( yes "" | git mergetool file1 file1 ) &&
-	( yes "" | git mergetool file2 "spaced name" >/dev/null 2>&1 ) &&
-	( yes "" | git mergetool subdir/file3 >/dev/null 2>&1 ) &&
-	( yes "d" | git mergetool file11 >/dev/null 2>&1 ) &&
-	( yes "d" | git mergetool file12 >/dev/null 2>&1 ) &&
-	( yes "l" | git mergetool submod >/dev/null 2>&1 ) &&
+	( yes "" | git mergetool file2 "spaced name" ) &&
+	( yes "" | git mergetool subdir/file3 ) &&
+	( yes "d" | git mergetool file11 ) &&
+	( yes "d" | git mergetool file12 ) &&
+	( yes "l" | git mergetool submod ) &&
+	test "$(cat file1)" = "master updated" &&
+	test "$(cat file2)" = "master new" &&
+	test "$(cat subdir/file3)" = "master new sub" &&
+	test "$(cat submod/bar)" = "branch1 submodule" &&
+	git commit -m "branch1 resolved with mergetool"
+'
+
+test_expect_success 'gui mergetool' '
+	test_config merge.guitool myguitool &&
+	test_config mergetool.myguitool.cmd "(printf \"gui \" && cat \"\$REMOTE\") >\"\$MERGED\"" &&
+	test_config mergetool.myguitool.trustExitCode true &&
+	test_when_finished "git reset --hard" &&
+	git checkout -b test$test_count branch1 &&
+	git submodule update -N &&
+	test_must_fail git merge master &&
+	( yes "" | git mergetool --gui both ) &&
+	( yes "" | git mergetool -g file1 file1 ) &&
+	( yes "" | git mergetool --gui file2 "spaced name" ) &&
+	( yes "" | git mergetool --gui subdir/file3 ) &&
+	( yes "d" | git mergetool --gui file11 ) &&
+	( yes "d" | git mergetool --gui file12 ) &&
+	( yes "l" | git mergetool --gui submod ) &&
+	test "$(cat file1)" = "gui master updated" &&
+	test "$(cat file2)" = "gui master new" &&
+	test "$(cat subdir/file3)" = "gui master new sub" &&
+	test "$(cat submod/bar)" = "branch1 submodule" &&
+	git commit -m "branch1 resolved with mergetool"
+'
+
+test_expect_success 'gui mergetool without merge.guitool set falls back to merge.tool' '
+	test_when_finished "git reset --hard" &&
+	git checkout -b test$test_count branch1 &&
+	git submodule update -N &&
+	test_must_fail git merge master &&
+	( yes "" | git mergetool --gui both ) &&
+	( yes "" | git mergetool -g file1 file1 ) &&
+	( yes "" | git mergetool --gui file2 "spaced name" ) &&
+	( yes "" | git mergetool --gui subdir/file3 ) &&
+	( yes "d" | git mergetool --gui file11 ) &&
+	( yes "d" | git mergetool --gui file12 ) &&
+	( yes "l" | git mergetool --gui submod ) &&
 	test "$(cat file1)" = "master updated" &&
 	test "$(cat file2)" = "master new" &&
 	test "$(cat subdir/file3)" = "master new sub" &&
@@ -153,15 +194,15 @@ test_expect_success 'mergetool crlf' '
 	# test_when_finished is LIFO.)
 	test_config core.autocrlf true &&
 	git checkout -b test$test_count branch1 &&
-	test_must_fail git merge master >/dev/null 2>&1 &&
-	( yes "" | git mergetool file1 >/dev/null 2>&1 ) &&
-	( yes "" | git mergetool file2 >/dev/null 2>&1 ) &&
-	( yes "" | git mergetool "spaced name" >/dev/null 2>&1 ) &&
-	( yes "" | git mergetool both >/dev/null 2>&1 ) &&
-	( yes "" | git mergetool subdir/file3 >/dev/null 2>&1 ) &&
-	( yes "d" | git mergetool file11 >/dev/null 2>&1 ) &&
-	( yes "d" | git mergetool file12 >/dev/null 2>&1 ) &&
-	( yes "r" | git mergetool submod >/dev/null 2>&1 ) &&
+	test_must_fail git merge master &&
+	( yes "" | git mergetool file1 ) &&
+	( yes "" | git mergetool file2 ) &&
+	( yes "" | git mergetool "spaced name" ) &&
+	( yes "" | git mergetool both ) &&
+	( yes "" | git mergetool subdir/file3 ) &&
+	( yes "d" | git mergetool file11 ) &&
+	( yes "d" | git mergetool file12 ) &&
+	( yes "r" | git mergetool submod ) &&
 	test "$(printf x | cat file1 -)" = "$(printf "master updated\r\nx")" &&
 	test "$(printf x | cat file2 -)" = "$(printf "master new\r\nx")" &&
 	test "$(printf x | cat subdir/file3 -)" = "$(printf "master new sub\r\nx")" &&
@@ -176,8 +217,8 @@ test_expect_success 'mergetool in subdir' '
 	git submodule update -N &&
 	(
 		cd subdir &&
-		test_must_fail git merge master >/dev/null 2>&1 &&
-		( yes "" | git mergetool file3 >/dev/null 2>&1 ) &&
+		test_must_fail git merge master &&
+		( yes "" | git mergetool file3 ) &&
 		test "$(cat file3)" = "master new sub"
 	)
 '
@@ -188,14 +229,14 @@ test_expect_success 'mergetool on file in parent dir' '
 	git submodule update -N &&
 	(
 		cd subdir &&
-		test_must_fail git merge master >/dev/null 2>&1 &&
-		( yes "" | git mergetool file3 >/dev/null 2>&1 ) &&
-		( yes "" | git mergetool ../file1 >/dev/null 2>&1 ) &&
-		( yes "" | git mergetool ../file2 ../spaced\ name >/dev/null 2>&1 ) &&
-		( yes "" | git mergetool ../both >/dev/null 2>&1 ) &&
-		( yes "d" | git mergetool ../file11 >/dev/null 2>&1 ) &&
-		( yes "d" | git mergetool ../file12 >/dev/null 2>&1 ) &&
-		( yes "l" | git mergetool ../submod >/dev/null 2>&1 ) &&
+		test_must_fail git merge master &&
+		( yes "" | git mergetool file3 ) &&
+		( yes "" | git mergetool ../file1 ) &&
+		( yes "" | git mergetool ../file2 ../spaced\ name ) &&
+		( yes "" | git mergetool ../both ) &&
+		( yes "d" | git mergetool ../file11 ) &&
+		( yes "d" | git mergetool ../file12 ) &&
+		( yes "l" | git mergetool ../submod ) &&
 		test "$(cat ../file1)" = "master updated" &&
 		test "$(cat ../file2)" = "master new" &&
 		test "$(cat ../submod/bar)" = "branch1 submodule" &&
@@ -209,9 +250,9 @@ test_expect_success 'mergetool skips autoresolved' '
 	git submodule update -N &&
 	test_must_fail git merge master &&
 	test -n "$(git ls-files -u)" &&
-	( yes "d" | git mergetool file11 >/dev/null 2>&1 ) &&
-	( yes "d" | git mergetool file12 >/dev/null 2>&1 ) &&
-	( yes "l" | git mergetool submod >/dev/null 2>&1 ) &&
+	( yes "d" | git mergetool file11 ) &&
+	( yes "d" | git mergetool file12 ) &&
+	( yes "l" | git mergetool submod ) &&
 	output="$(git mergetool --no-prompt)" &&
 	test "$output" = "No files need merging"
 '
@@ -259,9 +300,9 @@ test_expect_success 'mergetool skips resolved paths when rerere is active' '
 	rm -rf .git/rr-cache &&
 	git checkout -b test$test_count branch1 &&
 	git submodule update -N &&
-	test_must_fail git merge master >/dev/null 2>&1 &&
-	( yes "l" | git mergetool --no-prompt submod >/dev/null 2>&1 ) &&
-	( yes "d" "d" | git mergetool --no-prompt >/dev/null 2>&1 ) &&
+	test_must_fail git merge master &&
+	( yes "l" | git mergetool --no-prompt submod ) &&
+	( yes "d" "d" | git mergetool --no-prompt ) &&
 	git submodule update -N &&
 	output="$(yes "n" | git mergetool --no-prompt)" &&
 	test "$output" = "No files need merging"
@@ -369,9 +410,9 @@ test_expect_success 'deleted vs modified submodule' '
 	git checkout -b test$test_count.a test$test_count &&
 	test_must_fail git merge master &&
 	test -n "$(git ls-files -u)" &&
-	( yes "" | git mergetool file1 file2 spaced\ name subdir/file3 >/dev/null 2>&1 ) &&
-	( yes "" | git mergetool both >/dev/null 2>&1 ) &&
-	( yes "d" | git mergetool file11 file12 >/dev/null 2>&1 ) &&
+	( yes "" | git mergetool file1 file2 spaced\ name subdir/file3 ) &&
+	( yes "" | git mergetool both ) &&
+	( yes "d" | git mergetool file11 file12 ) &&
 	( yes "r" | git mergetool submod ) &&
 	rmdir submod && mv submod-movedaside submod &&
 	test "$(cat submod/bar)" = "branch1 submodule" &&
@@ -386,9 +427,9 @@ test_expect_success 'deleted vs modified submodule' '
 	git submodule update -N &&
 	test_must_fail git merge master &&
 	test -n "$(git ls-files -u)" &&
-	( yes "" | git mergetool file1 file2 spaced\ name subdir/file3 >/dev/null 2>&1 ) &&
-	( yes "" | git mergetool both >/dev/null 2>&1 ) &&
-	( yes "d" | git mergetool file11 file12 >/dev/null 2>&1 ) &&
+	( yes "" | git mergetool file1 file2 spaced\ name subdir/file3 ) &&
+	( yes "" | git mergetool both ) &&
+	( yes "d" | git mergetool file11 file12 ) &&
 	( yes "l" | git mergetool submod ) &&
 	test ! -e submod &&
 	output="$(git mergetool --no-prompt)" &&
@@ -400,9 +441,9 @@ test_expect_success 'deleted vs modified submodule' '
 	git submodule update -N &&
 	test_must_fail git merge test$test_count &&
 	test -n "$(git ls-files -u)" &&
-	( yes "" | git mergetool file1 file2 spaced\ name subdir/file3 >/dev/null 2>&1 ) &&
-	( yes "" | git mergetool both >/dev/null 2>&1 ) &&
-	( yes "d" | git mergetool file11 file12 >/dev/null 2>&1 ) &&
+	( yes "" | git mergetool file1 file2 spaced\ name subdir/file3 ) &&
+	( yes "" | git mergetool both ) &&
+	( yes "d" | git mergetool file11 file12 ) &&
 	( yes "r" | git mergetool submod ) &&
 	test ! -e submod &&
 	test -d submod.orig &&
@@ -416,9 +457,9 @@ test_expect_success 'deleted vs modified submodule' '
 	git submodule update -N &&
 	test_must_fail git merge test$test_count &&
 	test -n "$(git ls-files -u)" &&
-	( yes "" | git mergetool file1 file2 spaced\ name subdir/file3 >/dev/null 2>&1 ) &&
-	( yes "" | git mergetool both >/dev/null 2>&1 ) &&
-	( yes "d" | git mergetool file11 file12 >/dev/null 2>&1 ) &&
+	( yes "" | git mergetool file1 file2 spaced\ name subdir/file3 ) &&
+	( yes "" | git mergetool both ) &&
+	( yes "d" | git mergetool file11 file12 ) &&
 	( yes "l" | git mergetool submod ) &&
 	test "$(cat submod/bar)" = "master submodule" &&
 	git submodule update -N &&
@@ -440,9 +481,9 @@ test_expect_success 'file vs modified submodule' '
 	git checkout -b test$test_count.a branch1 &&
 	test_must_fail git merge master &&
 	test -n "$(git ls-files -u)" &&
-	( yes "" | git mergetool file1 file2 spaced\ name subdir/file3 >/dev/null 2>&1 ) &&
-	( yes "" | git mergetool both >/dev/null 2>&1 ) &&
-	( yes "d" | git mergetool file11 file12 >/dev/null 2>&1 ) &&
+	( yes "" | git mergetool file1 file2 spaced\ name subdir/file3 ) &&
+	( yes "" | git mergetool both ) &&
+	( yes "d" | git mergetool file11 file12 ) &&
 	( yes "r" | git mergetool submod ) &&
 	rmdir submod && mv submod-movedaside submod &&
 	test "$(cat submod/bar)" = "branch1 submodule" &&
@@ -456,9 +497,9 @@ test_expect_success 'file vs modified submodule' '
 	git checkout -b test$test_count.b test$test_count &&
 	test_must_fail git merge master &&
 	test -n "$(git ls-files -u)" &&
-	( yes "" | git mergetool file1 file2 spaced\ name subdir/file3 >/dev/null 2>&1 ) &&
-	( yes "" | git mergetool both >/dev/null 2>&1 ) &&
-	( yes "d" | git mergetool file11 file12 >/dev/null 2>&1 ) &&
+	( yes "" | git mergetool file1 file2 spaced\ name subdir/file3 ) &&
+	( yes "" | git mergetool both ) &&
+	( yes "d" | git mergetool file11 file12 ) &&
 	( yes "l" | git mergetool submod ) &&
 	git submodule update -N &&
 	test "$(cat submod)" = "not a submodule" &&
@@ -472,9 +513,9 @@ test_expect_success 'file vs modified submodule' '
 	git submodule update -N &&
 	test_must_fail git merge test$test_count &&
 	test -n "$(git ls-files -u)" &&
-	( yes "" | git mergetool file1 file2 spaced\ name subdir/file3 >/dev/null 2>&1 ) &&
-	( yes "" | git mergetool both >/dev/null 2>&1 ) &&
-	( yes "d" | git mergetool file11 file12 >/dev/null 2>&1 ) &&
+	( yes "" | git mergetool file1 file2 spaced\ name subdir/file3 ) &&
+	( yes "" | git mergetool both ) &&
+	( yes "d" | git mergetool file11 file12 ) &&
 	( yes "r" | git mergetool submod ) &&
 	test -d submod.orig &&
 	git submodule update -N &&
@@ -488,9 +529,9 @@ test_expect_success 'file vs modified submodule' '
 	git submodule update -N &&
 	test_must_fail git merge test$test_count &&
 	test -n "$(git ls-files -u)" &&
-	( yes "" | git mergetool file1 file2 spaced\ name subdir/file3 >/dev/null 2>&1 ) &&
-	( yes "" | git mergetool both>/dev/null 2>&1 ) &&
-	( yes "d" | git mergetool file11 file12 >/dev/null 2>&1 ) &&
+	( yes "" | git mergetool file1 file2 spaced\ name subdir/file3 ) &&
+	( yes "" | git mergetool both ) &&
+	( yes "d" | git mergetool file11 file12 ) &&
 	( yes "l" | git mergetool submod ) &&
 	test "$(cat submod/bar)" = "master submodule" &&
 	git submodule update -N &&
@@ -543,7 +584,7 @@ test_expect_success 'submodule in subdirectory' '
 	git add subdir/subdir_module &&
 	git commit -m "change submodule in subdirectory on test$test_count.b" &&
 
-	test_must_fail git merge test$test_count.a >/dev/null 2>&1 &&
+	test_must_fail git merge test$test_count.a &&
 	(
 		cd subdir &&
 		( yes "l" | git mergetool subdir_module )
@@ -554,7 +595,7 @@ test_expect_success 'submodule in subdirectory' '
 	git reset --hard &&
 	git submodule update -N &&
 
-	test_must_fail git merge test$test_count.a >/dev/null 2>&1 &&
+	test_must_fail git merge test$test_count.a &&
 	( yes "r" | git mergetool subdir/subdir_module ) &&
 	test "$(cat subdir/subdir_module/file15)" = "test$test_count.b" &&
 	git submodule update -N &&
@@ -641,7 +682,7 @@ test_expect_success 'filenames seen by tools start with ./' '
 	test_config mergetool.myecho.trustExitCode true &&
 	test_must_fail git merge master &&
 	git mergetool --no-prompt --tool myecho -- both >actual &&
-	grep ^\./both_LOCAL_ actual >/dev/null
+	grep ^\./both_LOCAL_ actual
 '
 
 test_lazy_prereq MKTEMP '
@@ -658,8 +699,8 @@ test_expect_success MKTEMP 'temporary filenames are used with mergetool.writeToT
 	test_config mergetool.myecho.trustExitCode true &&
 	test_must_fail git merge master &&
 	git mergetool --no-prompt --tool myecho -- both >actual &&
-	! grep ^\./both_LOCAL_ actual >/dev/null &&
-	grep /both_LOCAL_ actual >/dev/null
+	! grep ^\./both_LOCAL_ actual &&
+	grep /both_LOCAL_ actual
 '
 
 test_expect_success 'diff.orderFile configuration is honored' '
diff --git a/t/t7800-difftool.sh b/t/t7800-difftool.sh
index 480dd0633f..6bac9ed180 100755
--- a/t/t7800-difftool.sh
+++ b/t/t7800-difftool.sh
@@ -279,11 +279,27 @@ test_expect_success 'difftool + mergetool config variables' '
 	echo branch >expect &&
 	git difftool --no-prompt branch >actual &&
 	test_cmp expect actual &&
+	git difftool --gui --no-prompt branch >actual &&
+	test_cmp expect actual &&
 
 	# set merge.tool to something bogus, diff.tool to test-tool
 	test_config merge.tool bogus-tool &&
 	test_config diff.tool test-tool &&
 	git difftool --no-prompt branch >actual &&
+	test_cmp expect actual &&
+	git difftool --gui --no-prompt branch >actual &&
+	test_cmp expect actual &&
+
+	# set merge.tool, diff.tool to something bogus, merge.guitool to test-tool
+	test_config diff.tool bogus-tool &&
+	test_config merge.guitool test-tool &&
+	git difftool --gui --no-prompt branch >actual &&
+	test_cmp expect actual &&
+
+	# set merge.tool, diff.tool, merge.guitool to something bogus, diff.guitool to test-tool
+	test_config merge.guitool bogus-tool &&
+	test_config diff.guitool test-tool &&
+	git difftool --gui --no-prompt branch >actual &&
 	test_cmp expect actual
 '
 
@@ -715,4 +731,12 @@ test_expect_success 'outside worktree' '
 	test_cmp expect actual
 '
 
+test_expect_success 'difftool --gui, --tool and --extcmd are mutually exclusive' '
+	difftool_test_setup &&
+	test_must_fail git difftool --gui --tool=test-tool &&
+	test_must_fail git difftool --gui --extcmd=cat &&
+	test_must_fail git difftool --tool=test-tool --extcmd=cat &&
+	test_must_fail git difftool --gui --tool=test-tool --extcmd=cat
+'
+
 test_done