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

mercurial/obsolete.py

@@ -649,12 +649,13 b' def clearobscaches(repo):'
 def _computeobsoleteset(repo):
     """the set of obsolete revisions"""
     obs = set()
-    nm = repo.changelog.nodemap
+    getrev = repo.changelog.nodemap.get
+    getphase = repo._phasecache.phase
     for node in repo.obsstore.successors:
-        rev = nm.get(node)
-        if rev is not None:
+        rev = getrev(node)
+        if rev is not None and getphase(repo, rev):
             obs.add(rev)
-    return set(repo.revs('%ld - public()', obs))
+    return obs
 
 @cachefor('unstable')
 def _computeunstableset(repo):