CI: check that submodules point to correct branches (#956)

Co-authored-by: あく <alleteam@gmail.com>

.github/workflows/check_submodules.yml

@@ -0,0 +1,17 @@
+name: 'Check submodules'
+
+on:
+  pull_request:
+
+jobs:
+  protobuf:
+    runs-on: ubuntu-latest
+    steps:
+    - name: 'Checkout code'
+      uses: actions/checkout@v2
+    - name: 'Check submodule commit branch'
+      uses: jtmullen/submodule-branch-check-action@v1
+      with:
+        path: assets/protobuf
+        branch: dev
+        fetch_depth: 50