$ git clone -n https://src.fedoraproject.org/rpms/f23-backgrounds.git /var/lib/mock/f45-build-8581-391/root/chroot_tmpdir/scmroot/f23-backgrounds
Cloning into '/var/lib/mock/f45-build-8581-391/root/chroot_tmpdir/scmroot/f23-backgrounds'...
$ git fetch origin 1f4a0b0c007a6d7c8a3d9cb616c988e46e910e58:KOJI_FETCH_HEAD
From https://src.fedoraproject.org/rpms/f23-backgrounds
 * [new ref]         1f4a0b0c007a6d7c8a3d9cb616c988e46e910e58 -> KOJI_FETCH_HEAD
$ git reset --hard KOJI_FETCH_HEAD
HEAD is now at 1f4a0b0 Rebuilt for https://fedoraproject.org/wiki/Fedora_45_Mass_Rebuild