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>

replace-object.c

@@ -9,6 +9,7 @@
 #include "commit.h"
 
 static int register_replace_ref(const char *refname,
+				const char *referent UNUSED,
 				const struct object_id *oid,
 				int flag UNUSED,
 				void *cb_data)