Merge branch 'zh/push-to-delete-onelevel-ref'

Merge branch 'zh/push-to-delete-onelevel-ref'

"git push" has been taught to allow deletion of refs with one-level
names to help repairing a repository who acquired such a ref by
mistake.  In general, we don't encourage use of such a ref, and
creation or update to such a ref is rejected as before.

* zh/push-to-delete-onelevel-ref:
  push: allow delete single-level ref
  receive-pack: fix funny ref error messsage