$ git clone -n https://src.fedoraproject.org/rpms/f28-backgrounds.git /var/lib/mock/f45-build-8796-396/root/chroot_tmpdir/scmroot/f28-backgrounds
Cloning into '/var/lib/mock/f45-build-8796-396/root/chroot_tmpdir/scmroot/f28-backgrounds'...
$ git fetch origin 2b17e80f17470d4a230d549e75ddda86d459f700:KOJI_FETCH_HEAD
From https://src.fedoraproject.org/rpms/f28-backgrounds
 * [new ref]         2b17e80f17470d4a230d549e75ddda86d459f700 -> KOJI_FETCH_HEAD
$ git reset --hard KOJI_FETCH_HEAD
HEAD is now at 2b17e80 Rebuilt for https://fedoraproject.org/wiki/Fedora_45_Mass_Rebuild