refs: mark unused each_ref_fn parameters

Functions used with for_each_ref(), etc, need to conform to the
each_ref_fn interface. But most of them don't need every parameter;
let's annotate the unused ones to quiet -Wunused-parameter.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

replace-object.c

@@ -9,7 +9,8 @@
 static int register_replace_ref(struct repository *r,
 				const char *refname,
 				const struct object_id *oid,
-				int flag, void *cb_data)
+				int UNUSED(flag),
+				void *UNUSED(cb_data))
 {
 	/* Get sha1 from refname */
 	const char *slash = strrchr(refname, '/');