t4013: test diff-tree's --stdin commit formatting

Once upon a time, git-log was just "rev-list | diff-tree",
and we did not bother to test it separately. These days git-log
is implemented internally, but we want to make sure that the
rev-list to diff-tree pipeline continues to function. Let's
add a basic sanity test.

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

t/t4013-diff-various.sh

@@ -324,4 +324,14 @@ test_expect_success 'diff --cached -- file on unborn branch' '
 	test_cmp "$TEST_DIRECTORY/t4013/diff.diff_--cached_--_file0" result
 '
 
+test_expect_success 'diff-tree --stdin with log formatting' '
+	cat >expect <<-\EOF &&
+	Side
+	Third
+	Second
+	EOF
+	git rev-list master | git diff-tree --stdin --format=%s -s >actual &&
+	test_cmp expect actual
+'
+
 test_done