Add "--summary" option to git diff.

Remove the need to pipe git diff through git apply to
get the extended headers summary.

Signed-off-by: Sean Estabrooks <seanlkml@sympatico.ca>
Signed-off-by: Junio C Hamano <junkio@cox.net>

diff.h

@@ -31,7 +31,8 @@ struct diff_options {
 		 binary:1,
 		 full_index:1,
 		 silent_on_remove:1,
-		 find_copies_harder:1;
+		 find_copies_harder:1,
+		 summary:1;
 	int break_opt;
 	int detect_rename;
 	int line_termination;