Merge branch 'jx/do-not-crash-receive-pack-wo-head'

An attempt to delete a ref by pushing into a repositorywhose HEAD symbolic reference points at an unborn branch that cannot be created due to ref D/F conflict (e.g. refs/heads/a/b exists, HEAD points at refs/heads/a) failed. * jx/do-not-crash-receive-pack-wo-head: receive-pack: crash when checking with non-exist HEAD