ci: fetch parent commit for whitespace check

Author: yosef7

.github/workflows/site-validation.yml

@@ -22,6 +22,8 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v4
+        with:
+          fetch-depth: 2
       - uses: actions/setup-python@v5
         with:
           python-version: "3.12"