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