$ git clone -n https://src.fedoraproject.org/rpms/f21-backgrounds.git /var/lib/mock/f45-build-8630-392/root/chroot_tmpdir/scmroot/f21-backgrounds
Cloning into '/var/lib/mock/f45-build-8630-392/root/chroot_tmpdir/scmroot/f21-backgrounds'...
$ git fetch origin c2a8741de30e27026ed32191463ba853e5fc3b62:KOJI_FETCH_HEAD
From https://src.fedoraproject.org/rpms/f21-backgrounds
 * [new ref]         c2a8741de30e27026ed32191463ba853e5fc3b62 -> KOJI_FETCH_HEAD
$ git reset --hard KOJI_FETCH_HEAD
HEAD is now at c2a8741 Rebuilt for https://fedoraproject.org/wiki/Fedora_45_Mass_Rebuild