D5424: py3: use util.buffer instead of buffer in remotefilelog

[Yuya Nishihara]

> -        if isinstance(text, buffer):
> +        if isinstance(text, util.buffer):
>              text = str(text)

`util.buffer` isn't a type object on Python 3. Given `str(text)` didn't
explode, such cases wouldn't be covered by the tests.