$ git clone -n https://src.fedoraproject.org/rpms/ffmpeg.git /var/lib/mock/f42-build-869956-176924/root/chroot_tmpdir/scmroot/ffmpeg
Cloning into '/var/lib/mock/f42-build-869956-176924/root/chroot_tmpdir/scmroot/ffmpeg'...
$ git fetch origin d2ef10fe0c3d209e1245258fb340cf9880623fa7:KOJI_FETCH_HEAD
From https://src.fedoraproject.org/rpms/ffmpeg
 * [new ref]         d2ef10fe0c3d209e1245258fb340cf9880623fa7 -> KOJI_FETCH_HEAD
$ git reset --hard KOJI_FETCH_HEAD
HEAD is now at d2ef10f Rebase to 7.1 (rhbz#2273572)