$ git clone -n https://src.fedoraproject.org/rpms/ffmpeg.git /var/lib/mock/f42-build-3427134-45202/root/chroot_tmpdir/scmroot/ffmpeg
Cloning into '/var/lib/mock/f42-build-3427134-45202/root/chroot_tmpdir/scmroot/ffmpeg'...
$ git fetch origin 54e4bc559d6a5af15dc6836d8e5f28393a95aa6d:KOJI_FETCH_HEAD
From https://src.fedoraproject.org/rpms/ffmpeg
 * [new ref]         54e4bc559d6a5af15dc6836d8e5f28393a95aa6d -> KOJI_FETCH_HEAD
$ git reset --hard KOJI_FETCH_HEAD
HEAD is now at 54e4bc5 Rebuild for jpegxl (libjxl) 0.11.1