performance: speedup computation of hidden revisions In their current state, revset calls can be very costlys, as we test predicates on the entire repository. The hidden filter is very widely used, and needs to be very fast. This change drops revset calls in favor of direct revision manipulation. Performance test on my Mercurial checkout - 19857 total changesets, - 1584 obsolete changesets, - 13310 obsolescence markers. Before: ! hidden ! wall 0.077553 After this changes: ! hidden ! wall 0.011230 Performance test on a Mozilla central checkout: - 117293 total changesets, - 1 obsolete changeset, - 1 obsolescence marker. Before: ! hidden ! wall 0.389472 After: ! hidden ! wall 0.000079