Add the --submodule option to the diff option family

When you use the option --submodule=log you can see the submodule
summaries inlined in the diff, instead of not-quite-helpful SHA-1 pairs.

The format imitates what "git submodule summary" shows.

To do that, <path>/.git/objects/ is added to the alternate object
databases (if that directory exists).

This option was requested by Jens Lehmann at the GitTogether in Berlin.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Jens Lehmann <Jens.Lehmann@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

submodule.h

@@ -0,0 +1,8 @@
+#ifndef SUBMODULE_H
+#define SUBMODULE_H
+
+void show_submodule_summary(FILE *f, const char *path,
+		unsigned char one[20], unsigned char two[20],
+		const char *del, const char *add, const char *reset);
+
+#endif