gnomeos: Initial commit should be a compose

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.

gnomeos/yocto/commit-yocto-build.sh

@@ -43,4 +43,4 @@ BUILD_TIME=$(date -r $BUILD_TAR)
 
 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}