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