Merge branch 'kn/fetch-push-bulk-ref-update'

Merge branch 'kn/fetch-push-bulk-ref-update'

"git push" and "git fetch" are taught to update refs in batches to
gain performance.

* kn/fetch-push-bulk-ref-update:
  receive-pack: handle reference deletions separately
  refs/files: skip updates with errors in batched updates
  receive-pack: use batched reference updates
  send-pack: fix memory leak around duplicate refs
  fetch: use batched reference updates
  refs: add function to translate errors to strings