refs: add referent to each_ref_fn

Add a parameter to each_ref_fn so that callers to the ref APIs
that use this function as a callback can have acess to the
unresolved value of a symbolic ref.

Signed-off-by: John Cai <johncai86@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

commit-graph.c

@@ -1847,6 +1847,7 @@ struct refs_cb_data {
 };
 
 static int add_ref_to_set(const char *refname UNUSED,
+			  const char *referent UNUSED,
 			  const struct object_id *oid,
 			  int flags UNUSED, void *cb_data)
 {