invalidate_ref_cache(): rename function from invalidate_cached_refs()

invalidate_ref_cache(): rename function from invalidate_cached_refs()

It is the cache that is being invalidated, not the references, and the
new name makes this unambiguous.  Rename other items analogously:

* struct cached_refs -> struct ref_cache
* cached_refs (the variable) -> ref_cache
* clear_cached_refs() -> clear_ref_cache()
* create_cached_refs() -> create_ref_cache()
* get_cached_refs() -> get_ref_cache()

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