obsolete: skip 'changectx' usage in unstable computation

We simplify the unstable computation code, skipping the expensive creation of
changectx object. We focus on efficient set operation and revnumber centric
functions.

In my mercurial development repository, this provides a 3x speedup to the
function:

  before: 5.319 ms
  after:  1.844 ms

repo details:

  total changesets:       40886
  obsolete changesets:     7756
  mutable (not obsolete):   293
  unstable:                  30

mercurial/obsolete.py

@@ -1325,16 +1325,18 b' def _computeobsoleteset(repo):'
 @cachefor('unstable')
 def _computeunstableset(repo):
     """the set of non obsolete revisions with obsolete parents"""
-    revs = [(ctx.rev(), ctx) for ctx in
-            repo.set('(not public()) and (not obsolete())')]
-    revs.sort(key=lambda x:x[0])
+    pfunc = repo.changelog.parentrevs
+    mutable = _mutablerevs(repo)
+    obsolete = getrevs(repo, 'obsolete')
+    others = mutable - obsolete
     unstable = set()
-    for rev, ctx in revs:
+    for r in sorted(others):
         # A rev is unstable if one of its parent is obsolete or unstable
         # this works since we traverse following growing rev order
-        if any((x.obsolete() or (x.rev() in unstable))
-                for x in ctx.parents()):
-            unstable.add(rev)
+        for p in pfunc(r):
+            if p in obsolete or p in unstable:
+                unstable.add(r)
+                break
     return unstable
 
 @cachefor('suspended')