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

Documentation/fetch-options.txt

@@ -7,6 +7,10 @@
 	existing contents of `.git/FETCH_HEAD`.  Without this
 	option old data in `.git/FETCH_HEAD` will be overwritten.
 
+--atomic::
+	Use an atomic transaction to update local refs. Either all refs are
+	updated, or on error, no refs are updated.
+
 --depth=<depth>::
 	Limit fetching to the specified number of commits from the tip of
 	each remote branch history. If fetching to a 'shallow' repository