RFC: bitmap storage for hidden

[Jun Wu]

Excerpts from Pierre-Yves David's message of 2016-09-09 19:25:27 +0200:
> I'm still confused, the way we store hidden-ness and the way we compute 
> it are two different things. The current API definitely allow for other 
> way to marks things hidden. Sean even successfully implemented 
> experiment for hg-git a couple of month ago. So I don't see why this is 
> an argument in the current discussion.

I think it means the bitmap (only for hidden, not for phases) becomes the
persistent source of truth that cannot be easily discarded or rebuilt, and
can only be updated incrementally. For example, if I lose my obsstore or
Sean's hg-git hidden information, and when I run "hg log", I still get
previously hidden changesets right.

I think this enforces a good pattern that solves a lot of painful perf
issues and I'd like to see it happen.