$ git clone -n https://src.fedoraproject.org/rpms/libcxx.git /var/lib/mock/f41-build-810452-146696/root/chroot_tmpdir/scmroot/libcxx
Cloning into '/var/lib/mock/f41-build-810452-146696/root/chroot_tmpdir/scmroot/libcxx'...
$ git fetch origin 5e08997b4f72270d06d3d53a4797d209d2298336:KOJI_FETCH_HEAD
From https://src.fedoraproject.org/rpms/libcxx
 * [new ref]         5e08997b4f72270d06d3d53a4797d209d2298336 -> KOJI_FETCH_HEAD
$ git reset --hard KOJI_FETCH_HEAD
HEAD is now at 5e08997 18.1.6 Release