phases: incrementally update the phase sets when reasonable...
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".