Debug new workflow to check files

.github/workflows/check_diff.yml

@@ -15,9 +15,22 @@ jobs:
         uses: actions/checkout@v3
         with:
           fetch-depth: 0
+      - name: Define base and head refs
+        env:
+          BASE_REF: ${{ github.event.pull_request.base.ref }}
+          HEAD_REF: ${{ github.event.pull_request.head.ref }}
+        run: |
+          echo "Base ref: ${BASE_REF}"
+          echo "Head ref: ${HEAD_REF}"
+      - name: Add main repository as remote
+        run: |
+          git remote add main https://github.com/ECP-WarpX/WarpX.git
+      - name: Fetch base branch from main repository
+        run: |
+          git fetch main ${BASE_REF}
       - name: Get changed files
         run: |
-          git diff --name-only --diff-filter=ACMRTUXB ${GITHUB_BASE_REF}..${GITHUB_HEAD_REF} > check_diff.txt
+          git diff --name-only --diff-filter=ACMRTUXB main/${BASE_REF}..${HEAD_REF} > check_diff.txt
       - name: Check changed files
         run: |
           if grep -v -E "^(docs|\.github)/" check_diff.txt; then
@@ -26,4 +39,4 @@ jobs:
             echo "skip=true" >> ${GITHUB_OUTPUT}
           fi
     outputs:
-      skip: ${{ steps.check.outputs.skip_workflows }}
+      skip: ${{ steps.check.outputs.skip }}