Add a test that forbids merge commits

.github/workflows/ci.yaml

@@ -30,6 +30,9 @@ jobs:
     steps:
     - name: Checkout repository
       uses: actions/checkout@v2
+      with:
+        ref: ${{ github.event.pull_request.head.sha }}
+        fetch-depth: 5
 
     # We need to disable the existing toolchain to avoid updating rust-docs
     # which takes a long time. The fastest way to do this is to rename the