The internals of the refs API around the cached refs has been
streamlined.

* mh/separate-ref-cache:
  do_for_each_entry_in_dir(): delete function
  files_pack_refs(): use reference iteration
  commit_packed_refs(): use reference iteration
  cache_ref_iterator_begin(): make function smarter
  get_loose_ref_cache(): new function
  get_loose_ref_dir(): function renamed from get_loose_refs()
  do_for_each_entry_in_dir(): eliminate `offset` argument
  refs: handle "refs/bisect/" in `loose_fill_ref_dir()`
  ref-cache: use a callback function to fill the cache
  refs: record the ref_store in ref_cache, not ref_dir
  ref-cache: introduce a new type, ref_cache
  refs: split `ref_cache` code into separate files
  ref-cache: rename `remove_entry()` to `remove_entry_from_dir()`
  ref-cache: rename `find_ref()` to `find_ref_entry()`
  ref-cache: rename `add_ref()` to `add_ref_entry()`
  refs_verify_refname_available(): use function in more places
  refs_verify_refname_available(): implement once for all backends
  refs_ref_iterator_begin(): new function
  refs_read_raw_ref(): new function
  get_ref_dir(): don't call read_loose_refs() for "refs/bisect"