obsolete: speed up unstable computation

Speed up the computation of the unstable revset by using the not public()
revset. In another series of patches, we optimize the not public() revset and
together it leads to a 50-100x speedup on the computation of unstable() for
our big repos.

mercurial/obsolete.py

@@ -1110,13 +1110,17 b' def _computeobsoleteset(repo):'
 @cachefor('unstable')
 def _computeunstableset(repo):
     """the set of non obsolete revisions with obsolete parents"""
-    # revset is not efficient enough here
-    # we do (obsolete()::) - obsolete() by hand
-    obs = getrevs(repo, 'obsolete')
-    if not obs:
-        return set()
-    cl = repo.changelog
-    return set(r for r in cl.descendants(obs) if r not in obs)
+    revs = [(ctx.rev(), ctx) for ctx in
+            repo.set('(not public()) and (not obsolete())')]
+    revs.sort(key=lambda x:x[0])
+    unstable = set()
+    for rev, ctx in revs:
+        # A rev is unstable if one of its parent is obsolete or unstable
+        # this works since we traverse following growing rev order
+        if util.any((x.obsolete() or (x.rev() in unstable))
+                for x in ctx.parents()):
+            unstable.add(rev)
+    return unstable
 
 @cachefor('suspended')
 def _computesuspendedset(repo):