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