def _get_valid_ref(self) -> str | None:
        """
        Determines a valid Git reference for comparison.

        Returns:
            str | None: A valid Git reference or None if no valid reference is found.
        """
        current_branch = self._get_current_branch()
        default_branch = self._get_default_branch()

        ref_current_branch = f'origin/{current_branch}'
        ref_non_default_branch = f'$(git --no-pager merge-base HEAD "$(git --no-pager rev-parse --abbrev-ref origin/{default_branch})")'
        ref_default_branch = 'origin/' + default_branch
        ref_new_repo = '$(git --no-pager rev-parse --verify 4b825dc642cb6eb9a060e54bf8d69288fbee4904)'  # compares with empty tree

        refs = [
            ref_current_branch,
            ref_non_default_branch,
            ref_default_branch,
            ref_new_repo,
        ]
        for ref in refs:
            if self._verify_ref_exists(ref):
                return ref

        return None

        Determines a valid Git reference for comparison.

        Returns:
            str | None: A valid Git reference or None if no valid reference is found.
        
openhands/runtime/utils/git_handler.py