$ git clone -n https://src.fedoraproject.org/rpms/firefox.git /var/lib/mock/f43-build-58846484-6568586/root/chroot_tmpdir/scmroot/firefox
Cloning into '/var/lib/mock/f43-build-58846484-6568586/root/chroot_tmpdir/scmroot/firefox'...
$ git fetch origin 540d80f5e279bf400de134cd7dc7cfc64cbcf3b8:KOJI_FETCH_HEAD
From https://src.fedoraproject.org/rpms/firefox
 * [new ref]         540d80f5e279bf400de134cd7dc7cfc64cbcf3b8 -> KOJI_FETCH_HEAD
$ git reset --hard KOJI_FETCH_HEAD
HEAD is now at 540d80f Update 137.0.2