ci: check if there are commits to squash

we can then configure github to never merge when this doesn't pass

.github/workflows/conformance.yml

@@ -58,7 +58,7 @@ jobs:
             echo "Debug: ccache statistics (after the build):"
             ccache -s
             chmod -R ugo+rwx /tmp/KeySetCache
-      
+
       - name: Build compiler
         if: ${{ matrix.compiler == 'latest' }}
         uses: addnab/docker-run-action@v3
@@ -108,3 +108,15 @@ jobs:
             echo "Debug: ccache statistics (after the build):"
             ccache -s
             chmod -R ugo+rwx /tmp/KeySetCache
+
+  BlockMerge:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+        with:
+          fetch-depth: 0
+
+      - name: Check Commit to Squash
+        run: |
+          set -e
+          git log origin/${{ github.base_ref }}..origin/${{ github.head_ref }} --format=%s | ( ! grep -e "^f [0-9a-f]\+" -q )