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>

builtin/gc.c

@@ -836,6 +836,7 @@ struct cg_auto_data {
 };
 
 static int dfs_on_ref(const char *refname UNUSED,
+		      const char *referent UNUSED,
 		      const struct object_id *oid,
 		      int flags UNUSED,
 		      void *cb_data)