graph: fix coloring around octopus merges

When drawing the graph of an octopus merge, we draw a horizontal line
from parents 3 and above into the asterisk representing the commit. The
sections of this line should be colored to match the graph lines coming
in from above.

However, if the commit is not in the left-most column we do not take
into account the columns to the left of the commit when calculating
these colors. Fix this by adding the appropriate offset to the column
index used for calculating the color.

Signed-off-by: Hemmo Nieminen <hemmo.nieminen@iki.fi>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

graph.c

@@ -801,10 +801,10 @@ static int graph_draw_octopus_merge(struct git_graph *graph,
 	int num_dashes =
 		((graph->num_parents - dashless_commits) * 2) - 1;
 	for (i = 0; i < num_dashes; i++) {
-		col_num = (i / 2) + dashless_commits;
+		col_num = (i / 2) + dashless_commits + graph->commit_index;
 		strbuf_write_column(sb, &graph->new_columns[col_num], '-');
 	}
-	col_num = (i / 2) + dashless_commits;
+	col_num = (i / 2) + dashless_commits + graph->commit_index;
 	strbuf_write_column(sb, &graph->new_columns[col_num], '.');
 	return num_dashes + 1;
 }