phases: mark `phasecache.phaseroots` private We are about to change its content from nodeid to revnum. So anyone directly using the content might be in unexpected troubles. We start by making it private to explicitly break any such user (and discourage them to do so).

phases: check secret presence the right way during discovery There is an official function for this, lets use it. This will prevent the code to break in the future while we refactor the phase code.

phases: explicitly filter stripped revision at strip time Explicit is better than implicit. The current logic is bit subtle and fragile. It also get in the way of using something else than node-id as internal storage. We replace it with a more explicit filtering while striping.