D4750: filelog: stop proxying rawsize() (API)

[Yuya Nishihara]

>   > Apparently thg uses this to avoid loading the data into memory just to figure out the length like fctx.rawsize().  Any suggestions?
>   >
>   > https://groups.google.com/d/msg/thg-dev/6ekYID7ho_o/fbdd6rVDAwAJ
>   
>   
>   Just found this comment when going through mail backlog.
>   
>   Do we want to do anything about this before the 4.8 release?

No. I've updated the caller to directly access to _revlog.

> I'd really like to know why thg wants to access the size of a revision
> without accessing the revision text...

In order to decide whether diff should be generated. There's a "max size
to show diff" config knob, and we don't want to load large blob into memory.

What we need is a light-weight method to get approximate data size.