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>

submodule.h

@@ -3,6 +3,7 @@
 
 void show_submodule_summary(FILE *f, const char *path,
 		unsigned char one[20], unsigned char two[20],
+		unsigned dirty_submodule,
 		const char *del, const char *add, const char *reset);
 int is_submodule_modified(const char *path);