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".