Don't use rebase

.github/workflows/update-subtree.yml

@@ -70,10 +70,20 @@ jobs:
         cd ../verify-rust-std
         git remote add rust-filtered ../rust-tmp/
         git fetch rust-filtered
-        git checkout -b update-subtree/library rust-filtered/subtree/library
-        git rebase origin/subtree/library
+        git checkout -t -b update-subtree/library origin/subtree/library
+        SUBTREE_HEAD_MSG=$(git log --format=%s -n 1 origin/subtree/library)
+        UPSTREAM_FROM=$(git log --grep="${SUBTREE_HEAD_MSG}" -n 1 --format=%H rust-filtered/subtree/library)
+        UPSTREAM_HEAD=$(git log --format=%H -n 1 rust-filtered/subtree/library)
+        if [ "${UPSTREAM_HEAD}" = "${UPSTREAM_FROM}" ]; then
+          echo "Nothing to cherry-pick, ${UPSTREAM_FROM} matches ${UPSTREAM_HEAD} (${SUBTREE_HEAD_MSG})"
+          echo "MERGE_CONFLICTS=noop" >> $GITHUB_ENV
+        else
+          git cherry-pick ${UPSTREAM_FROM}..rust-filtered/subtree/library; then
+          echo "MERGE_CONFLICTS=maybe" >> $GITHUB_ENV
+        fi
 
     - name: Create Pull Request
+      if: ${{ env.MERGE_CONFLICTS != 'noop' }}
       uses: peter-evans/create-pull-request@v7
       with:
         title: 'Update subtree/library'
@@ -86,6 +96,7 @@ jobs:
         path: verify-rust-std
 
     - name: Merge subtree/library changes
+      if: ${{ env.MERGE_CONFLICTS != 'noop' }}
       run: |
         cd verify-rust-std
         SYNC_BRANCH="sync-$TOOLCHAIN_DATE"