Merge branch 'ps/update-ref-multi-transaction'

"git update-ref --stdin" learns to take multiple transactions in a
single session.

* ps/update-ref-multi-transaction:
  update-ref: disallow "start" for ongoing transactions
  p1400: use `git-update-ref --stdin` to test multiple transactions
  update-ref: allow creation of multiple transactions
  t1400: avoid touching refs on filesystem