Merge branch 'ps/fetch-atomic'

Merge branch 'ps/fetch-atomic'

"git fetch" can make two separate fetches, but ref updates coming
from them were in two separate ref transactions under "--atomic",
which has been corrected.

* ps/fetch-atomic:
  fetch: make `--atomic` flag cover pruning of refs
  fetch: make `--atomic` flag cover backfilling of tags
  refs: add interface to iterate over queued transactional updates
  fetch: report errors when backfilling tags fails
  fetch: control lifecycle of FETCH_HEAD in a single place
  fetch: backfill tags before setting upstream
  fetch: increase test coverage of fetches