Get a diff between running config and a proposed file.

def _get_diff(self):
        """Get a diff between running config and a proposed file."""
        diff = []
        self._create_sot_file()
        diff_out = self._send_command(
            "show diff rollback-patch file {} file {}".format(
                "sot_file", self.candidate_cfg
            ),
            raw_text=True,
        )
        try:
            diff_out = (
                diff_out.split("Generating Rollback Patch")[1]
                .replace("Rollback Patch is Empty", "")
                .strip()
            )
            for line in diff_out.splitlines():
                if line:
                    if line[0].strip() != "!" and line[0].strip() != ".":
                        diff.append(line.rstrip(" "))
        except (AttributeError, KeyError):
            raise ReplaceConfigException(
                "Could not calculate diff. It's possible the given file doesn't exist."
            )
        return "\n".join(diff)