$ git clone -n https://src.fedoraproject.org/rpms/rubygem-ref.git /var/lib/mock/f45-build-35235-694/root/chroot_tmpdir/scmroot/rubygem-ref
Cloning into '/var/lib/mock/f45-build-35235-694/root/chroot_tmpdir/scmroot/rubygem-ref'...
$ git fetch origin a06ee751a8b2d420f8943966455aa2fe87ed21a3:KOJI_FETCH_HEAD
From https://src.fedoraproject.org/rpms/rubygem-ref
 * [new ref]         a06ee751a8b2d420f8943966455aa2fe87ed21a3 -> KOJI_FETCH_HEAD
$ git reset --hard KOJI_FETCH_HEAD
HEAD is now at a06ee75 Rebuilt for https://fedoraproject.org/wiki/Fedora_45_Mass_Rebuild