fix: update verbose logging to match actual git strategy

- Ensure consistency between implementation (git diff HEAD~1 HEAD)
- and verbose logging message for maintenance branches

Author: markpollack

.github/scripts/test_discovery.py

@@ -180,7 +180,7 @@ def _detect_default_base(self) -> str:
 
         # Show the actual strategy being used
         if branch and branch.endswith('.x'):
-            return f"git show HEAD (maintenance branch {branch})"
+            return f"git diff HEAD~1 HEAD (maintenance branch {branch})"
         elif pr_base:
             return f"origin/{pr_base} (PR base)"
         elif branch: