refs: move the remaining ref module declarations to refs.h

Some functions from the refs module were still declared in cache.h.
Move them to refs.h.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/init-db.c

@@ -4,6 +4,7 @@
  * Copyright (C) Linus Torvalds, 2005
  */
 #include "cache.h"
+#include "refs.h"
 #include "builtin.h"
 #include "exec_cmd.h"
 #include "parse-options.h"