diff-highlight: test graphs with --color

Our tests send git's output directly to files or pipes, so
there will never be any color. Let's do at least one --color
test to make sure that we can handle this case (which we
currently can, but will be an easy thing to mess up when we
touch the graph code in a future patch).

We'll just cover the --graph case, since this is much more
complex than the earlier cases (i.e., if it manages to
highlight, then the non-graph case definitely would).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/diff-highlight/t/t9400-diff-highlight.sh

@@ -264,6 +264,15 @@ test_expect_success 'diff-highlight works with the --graph option' '
 	test_cmp graph.exp graph.act
 '
 
+# Just reuse the previous graph test, but with --color.  Our trimming
+# doesn't know about color, so just sanity check that something got
+# highlighted.
+test_expect_success 'diff-highlight works with color graph' '
+	git log --branches -p --date-order --graph --color |
+		"$DIFF_HIGHLIGHT" | trim_graph | left_trim >graph &&
+	grep "\[7m" graph
+'
+
 # Most combined diffs won't meet diff-highlight's line-number filter. So we
 # create one here where one side drops a line and the other modifies it. That
 # should result in a diff like: