This is just cleaner - we know where the ref came from, rather
than making it a copy, and also we can use it with the new
--recompose bits to keep adding to it.
ostree --repo=${OSTREE_REPO} commit --skip-if-unchanged -s "Build from OSTree ${OSTREE_VER}" -b "${BASE}" --tree=tar=${BUILD_TAR}
ostree --repo=${OSTREE_REPO} diff "${BASE}"^ "${BASE}" || true
-cp ${OSTREE_REPO}/refs/heads/${BASE} ${OSTREE_REPO}/refs/heads/${BUILDROOT}
+ostree --repo=${OSTREE_REPO} compose -s "Initial compose" -b ${BUILDROOT} ${BASE}