obsolete: speed up computation of bumped revset

Speed up the computation of the bumped revset by using the not public() revset.
In another patch series, we optimized the not public() revset. Together,
this cuts the cost of the computation of bumped() from 17% of the total time of
smartlog on our big repo to under 0.1% of the total time.

mercurial/obsolete.py

@@ -1139,11 +1139,10 b' def _computebumpedset(repo):'
     public = phases.public
     cl = repo.changelog
     torev = cl.nodemap.get
-    obs = getrevs(repo, 'obsolete')
-    for rev in repo:
+    for ctx in repo.set('(not public()) and (not obsolete())'):
+        rev = ctx.rev()
         # We only evaluate mutable, non-obsolete revision
-        if (public < phase(repo, rev)) and (rev not in obs):
-            node = cl.node(rev)
+        node = ctx.node()
             # (future) A cache of precursors may worth if split is very common
             for pnode in allprecursors(repo.obsstore, [node],
                                        ignoreflags=bumpedfix):