Merge branch 'hn/reftable-prep-part-2'

Further preliminary change to refs API.

* hn/reftable-prep-part-2:
  Make HEAD a PSEUDOREF rather than PER_WORKTREE.
  Modify pseudo refs through ref backend storage
  t1400: use git rev-parse for testing PSEUDOREF existence

diff --cc refs.c
Simple merge

diff --cc t/t1400-update-ref.sh
Simple merge