$ git clone -n https://src.fedoraproject.org/rpms/mock.git /var/lib/mock/f42-build-side-1-3498998-45964/root/chroot_tmpdir/scmroot/mock
Cloning into '/var/lib/mock/f42-build-side-1-3498998-45964/root/chroot_tmpdir/scmroot/mock'...
$ git fetch origin 6f17f9618692398b49a25ffebbf01b55228c1cfa:KOJI_FETCH_HEAD
From https://src.fedoraproject.org/rpms/mock
 * [new ref]         6f17f9618692398b49a25ffebbf01b55228c1cfa -> KOJI_FETCH_HEAD
$ git reset --hard KOJI_FETCH_HEAD
HEAD is now at 6f17f96 Update mock to 6.1-1