$ git clone -n https://src.fedoraproject.org/rpms/mkvtoolnix.git /var/lib/mock/f39-build-1661892-40666/root/chroot_tmpdir/scmroot/mkvtoolnix
Cloning into '/var/lib/mock/f39-build-1661892-40666/root/chroot_tmpdir/scmroot/mkvtoolnix'...
$ git fetch origin b9812d21a8d3b302782f6e977b9f15841023b8c4:KOJI_FETCH_HEAD
From https://src.fedoraproject.org/rpms/mkvtoolnix
 * [new ref]         b9812d21a8d3b302782f6e977b9f15841023b8c4 -> KOJI_FETCH_HEAD
$ git reset --hard KOJI_FETCH_HEAD
HEAD is now at b9812d2 Merge branch 'f38' into f39