ci: add diff whitespace check (#2572)

.github/workflows/lint.yml

@@ -13,6 +13,28 @@ jobs:
     steps:
       - name: Checkout
         uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6
+        with:
+          fetch-depth: 1
+
+      - name: Run git diff --check
+        shell: bash
+        env:
+          EVENT_NAME: ${{ github.event_name }}
+          PR_BASE_SHA: ${{ github.event.pull_request.base.sha }}
+          PUSH_BEFORE_SHA: ${{ github.event.before }}
+          GITHUB_SHA: ${{ github.sha }}
+        run: |
+          set -euo pipefail
+
+          if [ "$EVENT_NAME" = "pull_request" ]; then
+            git fetch --no-tags --depth=1 origin "+${PR_BASE_SHA}:refs/checks/pr-base"
+            git diff --check refs/checks/pr-base HEAD
+          elif [ "$PUSH_BEFORE_SHA" = "0000000000000000000000000000000000000000" ]; then
+            git diff-tree --check --no-commit-id --root -r "$GITHUB_SHA"
+          else
+            git fetch --no-tags --depth=1 origin "+${PUSH_BEFORE_SHA}:refs/checks/push-before"
+            git diff --check refs/checks/push-before HEAD
+          fi
 
       - name: Run markdownlint-cli2
         uses: DavidAnson/markdownlint-cli2-action@ded1f9488f68a970bc66ea5619e13e9b52e601cd # v23