$ git clone -n https://src.fedoraproject.org/rpms/libcxx.git /var/lib/mock/f42-build-864927-172657/root/chroot_tmpdir/scmroot/libcxx
Cloning into '/var/lib/mock/f42-build-864927-172657/root/chroot_tmpdir/scmroot/libcxx'...
$ git fetch origin 02a1dd5f345a241c3ab8e2a50131d6811172fd9a:KOJI_FETCH_HEAD
From https://src.fedoraproject.org/rpms/libcxx
 * [new ref]         02a1dd5f345a241c3ab8e2a50131d6811172fd9a -> KOJI_FETCH_HEAD
$ git reset --hard KOJI_FETCH_HEAD
HEAD is now at 02a1dd5 Update to 19.1.6