performance: speedup computation of obsolete revisions In their current state, revset calls can be very costly as we test predicates on the entire repository. As obsolete computation is used by the "hidden" filter, it needs to be very fast. This changet drops the revset call 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: ! obsolete ! wall 0.047041 After: ! obsolete ! wall 0.004590 Performance test on a Mozilla central checkout: - 117293 total changesets, - 1 obsolete changeset, - 1 obsolescence marker. Before: ! obsolete ! wall 0.001539 After: ! obsolete ! wall 0.000017