This function updates an existing checkout of source code.

def update(self, repo_dir, **kwargs):
        """This function updates an existing checkout of source code."""
        del kwargs
        rev = self._args.get("revision")
        if rev:
            return [{"args": ["git", "checkout", rev], "cwd": repo_dir}] + _ff_command(
                rev, repo_dir
            )
        return None