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/worktree.c

@@ -626,6 +626,7 @@ static void print_preparing_worktree_line(int detach,
  * Returns 0 on failure and non-zero on success.
  */
 static int first_valid_ref(const char *refname UNUSED,
+			   const char *referent UNUSED,
 			   const struct object_id *oid UNUSED,
 			   int flags UNUSED,
 			   void *cb_data UNUSED)