Fetch origin/master after checking out the repo

so we can perform git-diff to know which files has been changed in the
patch.

Author: amatsuda

.github/workflows/preview.yml

@@ -16,6 +16,13 @@ jobs:
     steps:
       - name: Checkout
         uses: actions/checkout@v4
+      - name: Fetch master
+        run: |
+          commits=${{ github.event.pull_request.commits }}
+          if [[ -n "$commits" ]]; then
+            # Prepare enough depth for diffs with master
+            git fetch --depth="$(( commits + 1 ))"
+          fi
 
       - uses: ruby/setup-ruby@v1
         with: