Merge branch 'ps/fetch-atomic' into ps/fetch-mirror-optim

Merge branch 'ps/fetch-atomic' into ps/fetch-mirror-optim

* 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