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>

pseudo-merge.c

@@ -201,6 +201,7 @@ void load_pseudo_merges_from_config(struct string_list *list)
 }
 
 static int find_pseudo_merge_group_for_ref(const char *refname,
+					   const char *referent UNUSED,
 					   const struct object_id *oid,
 					   int flags UNUSED,
 					   void *_data)