[RFC] speed up walk of working dir by 20-25%

[Florian Weimer]

* Vadim Gelfer:

> this patch uses c extension to make walk of working directory faster.
>
> approach is to read directory in c, use dirent.d_type to tell what type a
> file is without calling lstat. for "hg status", this reduces number of calls
> to lstat by 10% on kernel repo (no need to lstat directories). for "hg add",
> this makes all 20000 calls to lstat go away (no need to lstat anything).

You could also try to sort by the d_ino field.  It should help
cold-cache performance on most file systems.