Merge branch 'hn/refs-fetch-head-is-special'

Merge branch 'hn/refs-fetch-head-is-special'

The FETCH_HEAD is now always read from the filesystem regardless of
the ref backend in use, as its format is much richer than the
normal refs, and written directly by "git fetch" as a plain file..

* hn/refs-fetch-head-is-special:
  refs: read FETCH_HEAD and MERGE_HEAD generically
  refs: move gitdir into base ref_store
  refs: fix comment about submodule ref_stores
  refs: split off reading loose ref data in separate function