$ git clone -n https://src.fedoraproject.org/rpms/firefox.git /var/lib/mock/f39-build-1766393-41501/root/chroot_tmpdir/scmroot/firefox
Cloning into '/var/lib/mock/f39-build-1766393-41501/root/chroot_tmpdir/scmroot/firefox'...
$ git fetch origin ada751ca274f8865517fa00cd9c216423fec0c6a:KOJI_FETCH_HEAD
From https://src.fedoraproject.org/rpms/firefox
 * [new ref]         ada751ca274f8865517fa00cd9c216423fec0c6a -> KOJI_FETCH_HEAD
$ git reset --hard KOJI_FETCH_HEAD
HEAD is now at ada751c Updated to 120.0 Build 2