D7256: repoview: define filteredchangelog as a top-level (non-local) class

[Yuya Nishihara]

>   > And we might need a class cache like _filteredrepotypes to work around
>   > leaks of dynamically-created types.
>   
>   It seems like there's some caching in `_clcache` already. Maybe that's enough?

Maybe not because clcache won't live longer than the repo instance, but
I have no idea if that actually impact the leak seen in hgweb.