graph: tidy up display of left-skewed merges

Currently, when we display a merge whose first parent is already present
in a column to the left of the merge commit, we display the first parent
as a vertical pipe `|` in the GRAPH_POST_MERGE line and then immediately
enter the GRAPH_COLLAPSING state. The first-parent line tracks to the
left and all the other parent lines follow it; this creates a "kink" in
those lines:

        | *---.
        | |\ \ \
        |/ / / /
        | | | *

This change tidies the display of such commits such that if the first
parent appears to the left of the merge, we render it as a `/` and the
second parent as a `|`. This reduces the horizontal and vertical space
needed to render the merge, and makes the resulting lines easier to
read.

        | *-.
        |/|\ \
        | | | *

If the first parent is separated from the merge by several columns, a
horizontal line is drawn in a similar manner to how the GRAPH_COLLAPSING
state displays the line.

        | | | *-.
        | |_|/|\ \
        |/| | | | *

This effect is applied to both "normal" two-parent merges, and to
octopus merges. It also reduces the vertical space needed for pre-commit
lines, as the merge occupies one less column than usual.

        Before:         After:

        | *             | *
        | |\            | |\
        | | \           | * \
        | |  \          |/|\ \
        | *-. \
        | |\ \ \

Signed-off-by: James Coglan <jcoglan@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 39 insertions, 6 deletions

diff --git a/t/t4215-log-skewed-merges.sh b/t/t4215-log-skewed-merges.sh
index 4582ba066a..dc187b5caf 100755
--- a/t/t4215-log-skewed-merges.sh
+++ b/t/t4215-log-skewed-merges.sh
@@ -11,12 +11,8 @@ test_expect_success 'log --graph with merge fusing with its left and right neigh
 	| *   G
 	| |\
 	| | * F
-	| | |
-	| |  \
-	| *-. \   E
-	| |\ \ \
-	|/ / / /
-	| | | /
+	| * \   E
+	|/|\ \
 	| | |/
 	| | * D
 	| * | C
@@ -40,4 +36,41 @@ test_expect_success 'log --graph with merge fusing with its left and right neigh
 	test_cmp expect actual
 '
 
+test_expect_success 'log --graph with left-skewed merge' '
+	cat >expect <<-\EOF &&
+	*-----.   0_H
+	|\ \ \ \
+	| | | | * 0_G
+	| |_|_|/|
+	|/| | | |
+	| | | * \   0_F
+	| |_|/|\ \
+	|/| | | |/
+	| | | | * 0_E
+	| |_|_|/
+	|/| | |
+	| | * | 0_D
+	| | |/
+	| | * 0_C
+	| |/
+	|/|
+	| * 0_B
+	|/
+	* 0_A
+	EOF
+
+	git checkout --orphan 0_p && test_commit 0_A &&
+	git checkout -b 0_q 0_p && test_commit 0_B &&
+	git checkout -b 0_r 0_p &&
+	test_commit 0_C &&
+	test_commit 0_D &&
+	git checkout -b 0_s 0_p && test_commit 0_E &&
+	git checkout -b 0_t 0_p && git merge --no-ff 0_r^ 0_s -m 0_F &&
+	git checkout 0_p && git merge --no-ff 0_s -m 0_G &&
+	git checkout @^ && git merge --no-ff 0_q 0_r 0_t 0_p -m 0_H &&
+
+	git log --graph --pretty=tformat:%s | sed "s/ *$//" >actual &&
+	test_cmp expect actual
+'
+
 test_done