Add missing quotes

Author: tautschnig

.github/workflows/update-subtree.yml

@@ -56,7 +56,7 @@ jobs:
         # subtree/library as base.
         SUBTREE_HEAD=$(git log --format=%s -n 1 origin/subtree/library)
         echo "SUBTREE_HEAD: ${SUBTREE_HEAD}"
-        UPSTREAM_ONTO=$(git log --grep=$SUBTREE_HEAD -n 1 --format=%H upstream/master)
+        UPSTREAM_ONTO=$(git log --grep="$SUBTREE_HEAD" -n 1 --format=%H upstream/master)
         echo "UPSTREAM_ONTO: ${UPSTREAM_ONTO}"
 
         git checkout ${COMMIT_HASH}