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>

refs.h

@@ -298,7 +298,7 @@ struct ref_transaction;
  * arguments is only guaranteed to be valid for the duration of a
  * single callback invocation.
  */
-typedef int each_ref_fn(const char *refname,
+typedef int each_ref_fn(const char *refname, const char *referent,
 			const struct object_id *oid, int flags, void *cb_data);
 
 /*