fix: prioritize maintenance branch detection over explicit base_ref

- Ensure maintenance branches (.x) always use 'git show HEAD'
- Prevents fallback to git diff strategies that fail with shallow checkouts
- Fixes issue where --base parameter overrode maintenance branch logic

Author: markpollack

.github/scripts/test_discovery.py

@@ -62,7 +62,7 @@ def _get_changed_files(self, base_ref: Optional[str] = None) -> List[str]:
             pr_head = os.environ.get('GITHUB_HEAD_HEAD')   # PRs  
             branch = os.environ.get('GITHUB_REF_NAME')    # pushes
 
-            # For maintenance branches (cherry-picks), get files from the commit itself
+            # For maintenance branches (cherry-picks), always use git show HEAD regardless of base_ref
             if branch and branch.endswith('.x'):
                 # Maintenance branch - cherry-picks are single commits, just get files in this commit
                 cmd = ["git", "show", "--name-only", "--format=", "HEAD"]