$ git clone -n https://src.fedoraproject.org/rpms/mupdf.git /var/lib/mock/f42-build-864515-172387/root/chroot_tmpdir/scmroot/mupdf
Cloning into '/var/lib/mock/f42-build-864515-172387/root/chroot_tmpdir/scmroot/mupdf'...
$ git fetch origin 3b9e1e8cce15a6a2fe33cc91fcd47508aed1d9c2:KOJI_FETCH_HEAD
From https://src.fedoraproject.org/rpms/mupdf
 * [new ref]         3b9e1e8cce15a6a2fe33cc91fcd47508aed1d9c2 -> KOJI_FETCH_HEAD
$ git reset --hard KOJI_FETCH_HEAD
HEAD is now at 3b9e1e8 rebase to 1.25.2 (rhbz#2309092)