performance: speedup computation of suspended revisions

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

This change drops a 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:
  ! suspended
  ! wall 0.014319

After:
  ! suspended
  ! wall 0.009559

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

Before:
  ! suspended
  ! wall 0.033373

After:
  ! suspended
  ! wall 0.000053

mercurial/obsolete.py

@@ -671,7 +671,8 b' def _computeunstableset(repo):'
 @cachefor('suspended')
 def _computesuspendedset(repo):
     """the set of obsolete parents with non obsolete descendants"""
-    return set(repo.revs('obsolete() and obsolete()::unstable()'))
+    suspended = repo.changelog.ancestors(getrevs(repo, 'unstable'))
+    return set(r for r in getrevs(repo, 'obsolete') if r in suspended)
 
 @cachefor('extinct')
 def _computeextinctset(repo):