Merge branch 'ps/fetch-atomic'

"git fetch" learns to treat ref updates atomically in all-or-none fashion, just like "git push" does, with the new "--atomic" option. * ps/fetch-atomic: fetch: implement support for atomic reference updates fetch: allow passing a transaction to `s_update_ref()` fetch: refactor `s_update_ref` to use common exit path fetch: use strbuf to format FETCH_HEAD updates fetch: extract writing to FETCH_HEAD