performance: speedup computation of unstable revisions In their current state, revset calls can be very costly, as we test predicates on the entire repository. This change drops revset call in favor of direct testing of the phase of changesets. Performance test on my Mercurial checkout - 19857 total changesets, - 1584 obsolete changesets, - 13310 obsolescence markers. Before: ! unstable ! wall 0.017366 After this changes: ! unstable ! wall 0.008093 Performance test on a Mozilla central checkout: - 117293 total changesets, - 1 obsolete changeset, - 1 obsolescence marker. Before: ! unstable ! wall 0.045190 After: ! unstable ! wall 0.000032