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