diffcore-rename: somewhat optimized.

This changes diffcore-rename to reuse statistics information
gathered during similarity estimation, and updates the hashtable
implementation used to keep track of the statistics to be
denser.  This seems to give better performance.

Signed-off-by: Junio C Hamano <junkio@cox.net>

diffcore-break.c

@@ -68,6 +68,7 @@ static int should_break(struct diff_filespec *src,
 
 	if (diffcore_count_changes(src->data, src->size,
 				   dst->data, dst->size,
+				   NULL, NULL,
 				   0,
 				   &src_copied, &literal_added))
 		return 0;