D6417: context: get filesadded() and filesremoved() from changeset if configured

[Yuya Nishihara]

>   > I can't really comment on the storage format. I'm not keen on using extras
>   >  for this kind of stuff (including copies), but that seems be okay for
>   >  experiment.
>   
>   Do we have a better place for it?

I don't think so.

> What's your concern with using extras?
> Is it that we're storing information that could instead be calculated?

No, I don't care much about that.

> Or that a user could set the values?

Somewhat yes.

I just have a feeling that these copies/added/removed data are first class,
the repo can be somewhat corrupted if these data are wrong, which I don't think
are data meant to be stored in the extras.

Ideally, we can add some repo requirement and bump the revlog format to
store these data properly, but that's a big change. So I said storing in
extras seems okay for the time being.