RFC: bitmap storage for precursors and phases

[Jun Wu]

Excerpts from Stanislau Hlebik's message of 2017-02-17 16:06:33 +0000:
> This is implementation of two caches (nonpublic + precursor) using
> serialized sorted lists and sets
> https://bitbucket.org/stashlebik/hg/commits/99879579ac2848a2567810b677d8344150a7b319?at=hiddenbitmaps_lists

I had a quick look. Here are my suggestions:

1. Prefer caching at a lower-level, so they get more widely used.

   Practically, do not change localrepo.py, change phasecache.py and
   obsolete.py instead.

   I happened to do some refactoring in phasecache [1]. I'll send a V2 soon,
   since the change seems related.

   Note: phasecache is some C code so you need some data to prove the change
   is worthy.

2. The "Python set" serialization is already in repoview.py. Reuse it.

   Maybe move "_writehiddencache" to scmutil, or a new simple class.
   "tryreadcache" could be changed and then moved.

   Then the functions could be reused in obsolete.getrevs.

[1]: https://www.mercurial-scm.org/pipermail/mercurial-devel/2017-February/092817.html