Make diffcore_std only can run once before a diff_flush

When file renames/copies detection is turned on, the
second diffcore_std will degrade a 'C' pair to a 'R' pair.

And this may happen when we run 'git log --follow' with
hard copies finding. That is, the try_to_follow_renames()
will run diffcore_std to find the copies, and then
'git log' will issue another diffcore_std, which will reduce
'src->rename_used' and recognize this copy as a rename.
This is not what we want.

So, I think we really don't need to run diffcore_std more
than one time.

Signed-off-by: Bo Yang <struggleyb.nku@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diffcore.h

@@ -91,11 +91,13 @@ struct diff_queue_struct {
 	struct diff_filepair **queue;
 	int alloc;
 	int nr;
+	int run;
 };
 #define DIFF_QUEUE_CLEAR(q) \
 	do { \
 		(q)->queue = NULL; \
 		(q)->nr = (q)->alloc = 0; \
+		(q)->run = 0; \
 	} while(0);
 
 extern struct diff_queue_struct diff_queued_diff;