D7118: rust-dirstatemap: remove additional lookups in traverse

[Yuya Nishihara]

>   >> +++ b/mercurial/dirstate.py
>   >> @@ -919,6 +919,9 @@
>   >>
>   >>   matchalways = match.always()
>   >>   matchtdir = match.traversedir
>   >>   dmap = self._map
>   >>
>   >> +        if rustmod is not None:
>   >> +            dmap = self._map._rustmap
>   >
>   > If it's the same trick, can't it be abstracted away? `if rustmod` seems weird.
>   
>   The main issue I faced is that Python classes built from `rust-cpython` cannot be subclassed, so we have to resort to an additional level of indirection. IIRC, the main reason was that there was no way of enforcing a subclass to call `super()` which would result in the Rust class not being initialized properly. Or maybe I'm missing the point?

I mean there could be a unified `self._map._map` call if we need to take
the internal map from both py dirstatemap and rust dirstatemap.