RFC: bitmap storage for precursors and phases

[Stanislau Hlebik]

Excerpts from Sean Farley's message of 2017-02-13 18:30:25 -0800:
> Jun Wu <quark at fb.com> writes:
> 
> > Excerpts from Sean Farley's message of 2017-02-13 17:04:35 -0800:
> >> I was thinking about a more high-level approach (please feel free to
> >> pick apart):
> >> 
> >> r = repo.filtered("bitmap1")
> >> r2 = r.filtered("bitmap2")
> >> 
> >> So that r2 would be an intersection of bitmap1 and bitmap2 (haven't
> >> thought about a union nor the inverse).
> >
> > That does not conflict with my comments. It could be implemented as nested
> > filters, or flatten the bitmap by doing an "or" operation.
> 
> Righto. Just wanted to bring it up early before things are set in stone.

Current `repoview.filtered()` implementation can apply only one filter. I
think it will be error-prone to change it, won't it?