tests/test-status-inprocess.py.out

phases: add a getrevset method to phasecache

This is part of a refactoring that moves some phase query optimization from
revset.py to phases.py.

The motivation behind this was chg repo preloading - to make the obsstore
depend on less things (like the revset language). The refactoring also looks
good by itself - phasecache does not expose its private field "_phasesets"
via public methods and revset.py is accessing it in a hacky way.

This patch adds a "getrevset" method, which takes multiple phases and
returns a revset in an best-effort efficient way - for "public" phase, it
returns a lazy generatorset; for "draft" and "secret", it returns efficient
"baseset".

% creating repo
% add and commit
C test.py
% change
M test.py