Fix diff-cover no-merge-base error by fetching full git history

GitHub Actions shallow clone (fetch-depth: 1) prevented diff-cover from
finding the common ancestor between HEAD and origin/main. Setting
fetch-depth: 0 fetches the full history so diff-cover can compute the
merge base correctly.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: Abel Milash

.github/workflows/python-package.yml

@@ -18,6 +18,8 @@ jobs:
 
     steps:
     - uses: actions/checkout@v4
+      with:
+        fetch-depth: 0
 
     - name: Set up Python 3.12
       uses: actions/setup-python@v5