ci: notify when a PR has a merge conflict.

Adds a "merge conflict" label and posts a comment when a PR has a merge
conflict. See https://github.com/Marr11317/ConflictAdviser#limitations.

.github/workflows/test.yaml

@@ -10,6 +10,16 @@ on:
     - master
 
 jobs:
+  # https://github.com/Marr11317/ConflictAdviser
+  notify-merge-conflicts:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: Marr11317/ConflictAdviser@v1
+        with:
+          github_token: ${{ secrets.GITHUB_TOKEN }}
+          conflict_label: "merge conflict"
+          comment: 'Rebase needed'
+
   test:
     runs-on: ubuntu-latest
     container: ubuntu:20.04