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/fast-export.c

@@ -5,6 +5,7 @@
  */
 #include "builtin.h"
 #include "cache.h"
+#include "refs.h"
 #include "commit.h"
 #include "object.h"
 #include "tag.h"