Make compare_config() for merge look similar to replace config diff.

def _normalize_merge_diff(diff):
        """Make compare_config() for merge look similar to replace config diff."""
        new_diff = []
        for line in diff.splitlines():
            # Filter blank lines and prepend +sign
            if line.strip():
                new_diff.append("+" + line)
        if new_diff:
            new_diff.insert(
                0, "! incremental-diff failed; falling back to echo of merge file"
            )
        else:
            new_diff.append("! No changes specified in merge file.")
        return "\n".join(new_diff)