Simply abort if blocker is changed

.github/workflows/dependabot.yml

@@ -1,24 +1,25 @@
 name: Merge dependabot PRs automatically
-on:
-  pull_request_target:
-    paths:
-      - '**/*'
-      - '!rbs.gemspec'
-      - '!Gemfile'
-      - '!steep/Gemfile'
+on: pull_request_target
 
 permissions:
   pull-requests: write
   contents: write
 
+env:
+  blocker_files: rbs.gemspec Gemfile steep/Gemfile
+
 jobs:
   dependabot:
     runs-on: ubuntu-latest
+    if: ${{ github.actor == 'dependabot[bot]' }}
     steps:
-      - name: Hello
-        run: echo Hello world
+      - name: Checkout repository
+        uses: actions/checkout@v2
+        with:
+          fetch-depth: 0
+      - name: Abort if blocker files are changed
+        run: git diff --exit-code ${{ github.event.pull_request.base.sha }} ${{ github.event.pull_request.head.sha }} ${{ env.blocker_files }}
       - name: Enable auto-merge for Dependabot PRs
-        if: ${{ github.actor == 'dependabot[bot]' }}
         run: gh pr merge --auto --merge "$PR_URL"
         env:
           PR_URL: ${{github.event.pull_request.html_url}}