* mh/ref-api:
clear_ref_cache(): inline function
write_ref_sha1(): only invalidate the loose ref cache
clear_ref_cache(): extract two new functions
clear_ref_cache(): rename parameter
invalidate_ref_cache(): expose this function in the refs API
invalidate_ref_cache(): take the submodule as parameter
invalidate_ref_cache(): rename function from invalidate_cached_refs()