performance: speedup computation of unserved revisions In their current state, revset calls can be very costly, as we test predicates on the entire repository. The "unserved" filter is used in multiple applications, and in particular in some branch cache loading operations. We need to make it fast. This change drops revset calls in favor of direct testing of the phase of a changeset. Performance test on my Mercurial checkout - 19857 total changesets, - 1584 obsolete changesets, - 13310 obsolescence markers. Before: ! unserved ! wall 0.030477 After: ! unserved ! wall 0.011844 Performance test on a Mozilla central checkout: - 117293 total changesets, - 1 obsolete changeset, - 1 obsolescence marker. Before: ! unserved ! wall 0.111259 After: ! unserved ! wall 0.000084