diffcore_count_changes: pass diffcore_filespec

We may want to use richer information on the data we are dealing
with in this function, so instead of passing a buffer address
and length, just pass the diffcore_filespec structure.  Existing
callers always call this function with parameters taken from a
filespec anyway, so there is no functionality changes.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

diffcore-break.c

@@ -66,8 +66,7 @@ static int should_break(struct diff_filespec *src,
 	if (base_size < MINIMUM_BREAK_SIZE)
 		return 0; /* we do not break too small filepair */
 
-	if (diffcore_count_changes(src->data, src->size,
-				   dst->data, dst->size,
+	if (diffcore_count_changes(src, dst,
 				   NULL, NULL,
 				   0,
 				   &src_copied, &literal_added))