Add a macro DIFF_QUEUE_CLEAR.

Refactor the diff_queue_struct code, this macro help
to reset the structure.

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

diffcore-pickaxe.c

@@ -55,8 +55,7 @@ void diffcore_pickaxe(const char *needle, int opts)
 	int i, has_changes;
 	regex_t regex, *regexp = NULL;
 	struct diff_queue_struct outq;
-	outq.queue = NULL;
-	outq.nr = outq.alloc = 0;
+	DIFF_QUEUE_CLEAR(&outq);
 
 	if (opts & DIFF_PICKAXE_REGEX) {
 		int err;