tests: use 'test_must_be_empty' instead of 'test_cmp <empty> <out>'

Using 'test_must_be_empty' is shorter and more idiomatic than

  >empty &&
  test_cmp empty out

as it saves the creation of an empty file.  Furthermore, sometimes the
expected empty file doesn't have such a descriptive name like 'empty',
and its creation is far away from the place where it's finally used
for comparison (e.g. in 't7600-merge.sh', where two expected empty
files are created in the 'setup' test, but are used only about 500
lines later).

These cases were found by instrumenting 'test_cmp' to error out the
test script when it's used to compare empty files, and then converted
manually.

Note that even after this patch there still remain a lot of cases
where we use 'test_cmp' to check empty files:

  - Sometimes the expected output is not hard-coded in the test, but
    'test_cmp' is used to ensure that two similar git commands produce
    the same output, and that output happens to be empty, e.g. the
    test 'submodule update --merge  - ignores --merge  for new
    submodules' in 't7406-submodule-update.sh'.

  - Repetitive common tasks, including preparing the expected results
    and running 'test_cmp', are often extracted into a helper
    function, and some of this helper's callsites expect no output.

  - For the same reason as above, the whole 'test_expect_success'
    block is within a helper function, e.g. in 't3070-wildmatch.sh'.

  - Or 'test_cmp' is invoked in a loop, e.g. the test 'cvs update
    (-p)' in 't9400-git-cvsserver-server.sh'.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 3 insertions, 6 deletions

diff --git a/t/t7600-merge.sh b/t/t7600-merge.sh
index 6736d8d131..106148254d 100755
--- a/t/t7600-merge.sh
+++ b/t/t7600-merge.sh
@@ -38,7 +38,6 @@ printf '%s\n' '1 X' 2 3 4 5 6 7 8 9 >result.1
 printf '%s\n' '1 X' 2 3 4 '5 X' 6 7 8 9 >result.1-5
 printf '%s\n' '1 X' 2 3 4 '5 X' 6 7 8 '9 X' >result.1-5-9
 printf '%s\n' 1 2 3 4 5 6 7 8 '9 Z' >result.9z
->empty
 
 create_merge_msgs () {
 	echo "Merge tag 'c2'" >msg.1-5 &&
@@ -58,8 +57,6 @@ create_merge_msgs () {
 		echo &&
 		git log --no-merges ^HEAD c2 c3
 	} >squash.1-5-9 &&
-	: >msg.nologff &&
-	: >msg.nolognoff &&
 	{
 		echo "* tag 'c3':" &&
 		echo "  commit 3"
@@ -519,7 +516,7 @@ test_expect_success 'tolerate unknown values for merge.ff' '
 	test_tick &&
 	git merge c1 2>message &&
 	verify_head "$c1" &&
-	test_cmp empty message
+	test_must_be_empty message
 '
 
 test_expect_success 'combining --squash and --no-ff is refused' '
@@ -551,13 +548,13 @@ test_expect_success 'merge log message' '
 	git reset --hard c0 &&
 	git merge --no-log c2 &&
 	git show -s --pretty=format:%b HEAD >msg.act &&
-	test_cmp msg.nologff msg.act &&
+	test_must_be_empty msg.act &&
 
 	git reset --hard c0 &&
 	test_config branch.master.mergeoptions "--no-ff" &&
 	git merge --no-log c2 &&
 	git show -s --pretty=format:%b HEAD >msg.act &&
-	test_cmp msg.nolognoff msg.act &&
+	test_must_be_empty msg.act &&
 
 	git merge --log c3 &&
 	git show -s --pretty=format:%b HEAD >msg.act &&