performance: speedup computation of mutable revisions In their current state, revset calls can be very costly, as we test predicates on the entire repository. The "mutable" filter is used during branch cache loading operation. 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, - 1646 mutable revision Before: ! mutable ! wall 0.032405 After: ! mutable ! wall 0.001469 Performance test on a Mozilla central checkout: - 117293 total changesets, - 1 mutable changeset, Before: ! mutable ! wall 0.188636 After: ! mutable ! wall 0.000022