graph: example of graph output that can be simplified

The commits following this one introduce a series of improvements to the
layout of graphs, tidying up a few edge cases, namely:

- merge whose first parent fuses with an existing column to the left
- merge whose last parent fuses with its immediate neighbor on the right
- edges that collapse to the left above and below a commit line

This test case exemplifies these cases and provides a motivating example
of the kind of history I'm aiming to clear up.

The first parent of merge E is the same as the parent of H, so those
edges fuse together.

        * H
        |
        | *-.   E
        | |\ \
        |/ / /
        |
        * B

We can "skew" the display of this merge so that it doesn't introduce
additional columns that immediately collapse:

        * H
        |
        | *   E
        |/|\
        |
        * B

The last parent of E is D, the same as the parent of F which is the edge
to the right of the merge.

            * F
            |
             \
          *-. \   E
          |\ \ \
         / / / /
            | /
            |/
            * D

The two edges leading to D could be fused sooner: rather than expanding
the F edge around the merge and then letting the edges collapse, the F
edge could fuse with the E edge in the post-merge line:

            * F
            |
             \
          *-. | E
          |\ \|
         / / /
            |
            * D

If this is combined with the "skew" effect above, we get a much cleaner
graph display for these edges:

            * F
            |
          * | E
         /|\|
            |
            * D

Finally, the edge leading from C to A appears jagged as it passes
through the commit line for B:

        | * | C
        | |/
        * | B
        |/
        * A

This can be smoothed out so that such edges are easier to read:

        | * | C
        | |/
        * / B
        |/
        * A

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

t/t4215-log-skewed-merges.sh

@@ -0,0 +1,43 @@
+#!/bin/sh
+
+test_description='git log --graph of skewed merges'
+
+. ./test-lib.sh
+
+test_expect_success 'log --graph with merge fusing with its left and right neighbors' '
+	cat >expect <<-\EOF &&
+	*   H
+	|\
+	| *   G
+	| |\
+	| | * F
+	| | |
+	| |  \
+	| *-. \   E
+	| |\ \ \
+	|/ / / /
+	| | | /
+	| | |/
+	| | * D
+	| * | C
+	| |/
+	* | B
+	|/
+	* A
+	EOF
+
+	git checkout --orphan _p &&
+	test_commit A &&
+	test_commit B &&
+	git checkout -b _q @^ && test_commit C &&
+	git checkout -b _r @^ && test_commit D &&
+	git checkout _p && git merge --no-ff _q _r -m E &&
+	git checkout _r && test_commit F &&
+	git checkout _p && git merge --no-ff _r -m G &&
+	git checkout @^^ && git merge --no-ff _p -m H &&
+
+	git log --graph --pretty=tformat:%s | sed "s/ *$//" >actual &&
+	test_cmp expect actual
+'
+
+test_done