Performs argument glob expansion on an argument string.
        Returns a list of strings.

def _expand_argument(self, arg):
        """Performs argument glob expansion on an argument string.
        Returns a list of strings.
        """
        if isinstance(arg, R): return [arg.str]
        scope = self._scope or self._exports
        arg = arg % scope
        arg = os.path.expanduser(arg)
        res = glob.glob(self._expand_path(arg))
        if not res: return [arg]
        if self._cwd != "/":
            for idx in xrange(0, len(res)):
                if res[idx].startswith(self._cwd + "/"):
                    res[idx] = "./" + res[idx][len(self._cwd + "/"):]
        return res