$ git clone -n https://src.fedoraproject.org/rpms/libcxx.git /var/lib/mock/f41-build-855701-167312/root/chroot_tmpdir/scmroot/libcxx
Cloning into '/var/lib/mock/f41-build-855701-167312/root/chroot_tmpdir/scmroot/libcxx'...
$ git fetch origin 6087748767609f27584419debb6e056034283470:KOJI_FETCH_HEAD
From https://src.fedoraproject.org/rpms/libcxx
 * [new ref]         6087748767609f27584419debb6e056034283470 -> KOJI_FETCH_HEAD
$ git reset --hard KOJI_FETCH_HEAD
HEAD is now at 6087748 Update to 19.1.0