graph: fix collapse of multiple edges

This fix resolves the previously-added test_expect_failure in
t4215-log-skewed-merges.sh.

The issue lies in the "else" condition while updating the mapping
inside graph_output_collapsing_line(). In 0f0f389 (graph: tidy up
display of left-skewed merges, 2019-10-15), the output of left-
skewed merges was changed to allow an immediate horizontal edge in
the first parent, output by graph_output_post_merge_line() instead
of by graph_output_collapsing_line(). This condensed the first line
behavior as follows:

Before 0f0f389:

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

After 0f0f389:

	| | | | | | *
	| |_|_|_|_|/|\
	|/| | | | |/ /
	| | | | |/| /

However, a very subtle issue arose when the second and third parent
edges are collapsed in later steps. The second parent edge is now
immediately adjacent to a vertical edge. This means that the
condition

	} else if (graph->mapping[i - 1] < 0) {

in graph_output_collapsing_line() evaluates as false. The block for
this condition was the only place where we connected the target
column with the current position with horizontal edge markers.

In this case, the final "else" block is run, and the edge is marked
as horizontal, but did not back-fill the blank columns between the
target and the current edge. Since the second parent edge is marked
as horizontal, the third parent edge is not marked as horizontal.
This causes the output to continue as follows:

Before this change:

	| | | | | | *
	| |_|_|_|_|/|\
	|/| | | | |/ /
	| | | | |/| /
	| | | |/| |/
	| | |/| |/|
	| |/| |/| |
	| | |/| | |

By adding the logic for "filling" a horizontal edge between the
target column and the current column, we are able to resolve the
issue.

After this change:

	| | | | | | *
	| |_|_|_|_|/|\
	|/| | | | |/ /
	| | |_|_|/| /
	| |/| | | |/
	| | | |_|/|
	| | |/| | |

This output properly matches the expected blend of the edge
behavior before 0f0f389 and the merge commit rendering from
0f0f389.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

graph.c

@@ -1233,8 +1233,14 @@ static void graph_output_collapsing_line(struct git_graph *graph, struct graph_l
 			 * prevent any other edges from moving
 			 * horizontally.
 			 */
-			if (horizontal_edge == -1)
-				horizontal_edge = i;
+			if (horizontal_edge == -1) {
+				int j;
+				horizontal_edge_target = target;
+				horizontal_edge = i - 1;
+
+				for (j = (target * 2) + 3; j < (i - 2); j += 2)
+					graph->mapping[j] = target;
+			}
 		}
 	}
 

t/t4215-log-skewed-merges.sh

@@ -311,7 +311,7 @@ test_expect_success 'log --graph with multiple tips and colors' '
 	test_cmp expect.colors actual.colors
 '
 
-test_expect_failure 'log --graph with multiple tips' '
+test_expect_success 'log --graph with multiple tips' '
 	git checkout --orphan 7_1 &&
 	test_commit 7_A &&
 	test_commit 7_B &&