$ git clone -n https://src.fedoraproject.org/rpms/libcxx.git /var/lib/mock/f41-build-side-1-2751712-31120/root/chroot_tmpdir/scmroot/libcxx
Cloning into '/var/lib/mock/f41-build-side-1-2751712-31120/root/chroot_tmpdir/scmroot/libcxx'...
$ git fetch origin b68a5b693f5ef402260a5c861e4a8f6ab64023b9:KOJI_FETCH_HEAD
From https://src.fedoraproject.org/rpms/libcxx
 * [new ref]         b68a5b693f5ef402260a5c861e4a8f6ab64023b9 -> KOJI_FETCH_HEAD
$ git reset --hard KOJI_FETCH_HEAD
HEAD is now at b68a5b6 Update to 19.1.0