Merge branch 'ps/not-so-many-refs-are-special'

Merge branch 'ps/not-so-many-refs-are-special'

Define "special ref" as a very narrow set that consists of
FETCH_HEAD and MERGE_HEAD, and clarify everything else that used to
be classified as such are actually just pseudorefs.

* ps/not-so-many-refs-are-special:
  Documentation: add "special refs" to the glossary
  refs: redefine special refs
  refs: convert MERGE_AUTOSTASH to become a normal pseudo-ref
  sequencer: introduce functions to handle autostashes via refs
  refs: convert AUTO_MERGE to become a normal pseudo-ref
  sequencer: delete REBASE_HEAD in correct repo when picking commits
  sequencer: clean up pseudo refs with REF_NO_DEREF