Support transactional symbolic reference updates in git-update-ref(1)

To update HEAD in Gitaly with transaction support, we manually create a HEAD.lock file and modify the $GIT_DIR/HEAD file. This works with the file-backend, but we don't support modifying the $GIT_DIR/HEAD in the reftable backend.

Currently, git-symbolic-ref doesn't have transaction support and git-update-ref doesn't support symbolic updates. The easiest path would be to add symbolic reference updates to git-update-ref.