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

builtin/receive-pack.c

@@ -1460,8 +1460,10 @@ static const char *update(struct command *cmd, struct shallow_info *si)
 		find_shared_symref(worktrees, "HEAD", name);
 
 	/* only refs/... are allowed */
-	if (!starts_with(name, "refs/") || check_refname_format(name + 5, 0)) {
-		rp_error("refusing to create funny ref '%s' remotely", name);
+	if (!starts_with(name, "refs/") ||
+	    check_refname_format(name + 5, is_null_oid(new_oid) ?
+				 REFNAME_ALLOW_ONELEVEL : 0)) {
+		rp_error("refusing to update funny ref '%s' remotely", name);
 		ret = "funny refname";
 		goto out;
 	}