performance: speedup computation of extinct revisions

In their current state, revset calls can be very costly, as we test
predicates on the entire repository.

This change drops revset calls 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:
  ! extinct
  ! wall 0.015124

After:
  ! extinct
  ! wall 0.009424

Performance test on a Mozilla central checkout:
- 117293 total changesets,
- 1 obsolete changeset,
- 1 obsolescence marker.

Before:
  ! extinct
  ! wall 0.032844

After:
  ! extinct
  ! wall 0.000066

mercurial/obsolete.py

@@ -677,7 +677,7 b' def _computesuspendedset(repo):'
 @cachefor('extinct')
 def _computeextinctset(repo):
     """the set of obsolete parents without non obsolete descendants"""
-    return set(repo.revs('obsolete() - obsolete()::unstable()'))
+    return getrevs(repo, 'obsolete') - getrevs(repo, 'suspended')
 
 
 @cachefor('bumped')