Clone the given url and checkout the specific ref.

def clone(self, repo, ref, deps=()):
        """Clone the given url and checkout the specific ref."""

        if os.path.isdir(repo):
            repo = os.path.abspath(repo)

        def clone_strategy(directory):
            env = git.no_git_env()

            def _git_cmd(*args):
                cmd_output('git', *args, cwd=directory, env=env)

            _git_cmd('init', '.')
            _git_cmd('remote', 'add', 'origin', repo)

            try:
                self._shallow_clone(ref, _git_cmd)
            except CalledProcessError:
                self._complete_clone(ref, _git_cmd)

        return self._new_repo(repo, ref, deps, clone_strategy)