phases: make advance/retractboundary() atomic Before this, if advanceboundary() failed after updating some roots but before calling retractboundary(), the phase cache would be left in an invalid state, marked dirty, and written as such. This patch approach is to turn advance/retractboundary() into phasecache methods, then operate on copies and merge them back on success. With the same technique, we can ensure the atomicity of combinations of advance/retractboundary() calls, like those performed in changegroup handling code.

phases: introduce phasecache The original motivation was changectx.phase() had special logic to correctly lookup in repo._phaserev, including invalidating it when necessary. And at other places, repo._phaserev was accessed directly. This led to the discovery that phases state including _phaseroots, _phaserev and _dirtyphase was manipulated in localrepository.py, phases.py, repair.py, etc. phasecache helps encapsulating that. This patch replaces all phase state in localrepo with phasecache and adjust related code except for advance/retractboundary() in phases. These still access to phasecache internals directly. This will be addressed in a followup.

mq: introduce mq.check setting When: [mq] check = True is set, qpush, qpop and qgoto behave as if -c/--check were passed. If incompatible options like -f/--force or --exact are set, this setting is ignored. This setting enables what many users expect mq default behaviour to be.