Teach diff --submodule that modified submodule directory is dirty

Since commit 8e08b4 git diff does append "-dirty" to the work tree side
if the working directory of a submodule contains new or modified files.
Lets do the same when the --submodule option is used.

Signed-off-by: Jens Lehmann <Jens.Lehmann@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff.c

@@ -1615,7 +1615,7 @@ static void builtin_diff(const char *name_a,
 		const char *del = diff_get_color_opt(o, DIFF_FILE_OLD);
 		const char *add = diff_get_color_opt(o, DIFF_FILE_NEW);
 		show_submodule_summary(o->file, one ? one->path : two->path,
-				one->sha1, two->sha1,
+				one->sha1, two->sha1, two->dirty_submodule,
 				del, add, reset);
 		return;
 	}