phases: incrementally update the phase sets when reasonable When the amount of manual walking is small, we update the phases set manually instead of computing them from scratch. This should help small update. The next changesets will make this used more often by reducing the amount of full invalidation we do on roots upgrade. The criteria for using an incremental upgrade are arbitrary, however, it "should never hurt".

phasees: properly shallow caopy the phase sets dictionary We are about to increments the set more incrementally in some case, so we need to make a proper shallow copy of it.