$ git clone -n https://src.fedoraproject.org/rpms/gnome-backgrounds.git /var/lib/mock/f39-build-1462681-33355/root/chroot_tmpdir/scmroot/gnome-backgrounds
Cloning into '/var/lib/mock/f39-build-1462681-33355/root/chroot_tmpdir/scmroot/gnome-backgrounds'...
$ git fetch origin fa4419ec2833f9f524309fac4baff460f5bfbcc3:KOJI_FETCH_HEAD
From https://src.fedoraproject.org/rpms/gnome-backgrounds
 * [new ref]         fa4419ec2833f9f524309fac4baff460f5bfbcc3 -> KOJI_FETCH_HEAD
$ git reset --hard KOJI_FETCH_HEAD
HEAD is now at fa4419e Update to 45.0