verify that postgres has not changed

.github/workflows/tests.yml

@@ -667,6 +667,34 @@ jobs:
         path: .tmp
         retention-days: 1
 
+  pr-test:
+    # general PR tests
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v3
+      with:
+        fetch-depth: 0
+        submodules: false
+
+    - name: Verify that postgres/ was not changed unintentionally
+      run: |
+        title="${{ github.event.pull_request.title }}"
+        required_prefix="Update bundled PostgreSQL"
+
+        if [[ $title == $required_prefix* ]];
+          exit 0
+        fi
+
+        git diff --quiet \
+          ${{ github.event.pull_request.base.sha }} \
+          ${{ github.event.pull_request.head.sha }}
+        if [ $? != 0 ]; then
+          echo "postgres/ submodule has been changed,"\
+            "but PR title does not indicate that"
+          echo "(it should start with '$required_prefix')"
+          exit 1
+        fi
+
   test-conclusion:
     needs: [cargo-test, python-test, collect]
     runs-on: ubuntu-latest