Make invalidate_ref_cache() an official part of the refs API. It is
currently a fact of life that code outside of refs.c mucks about with
references. This change gives such code a way of informing the refs
module that it should no longer trust its cache.
Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu> Signed-off-by: Junio C Hamano <gitster@pobox.com>