obsolete: drop usage of changectx in '_computephasedivergentset'

Changectx are expensive and not needed there. The use of `repo.set` denote old
code that predate the introduction of `repo.revs` that we now use.

On my mercurial repository 495 draft:

    before: 0.010275 second
    after:  0.008832 second

On a mercurial repository with 115973 draft:

    before: 0.899255 second
    after:  0.397131 second

mercurial/obsolete.py

@@ -994,10 +994,10 b' def _computephasedivergentset(repo):'
     public = phases.public
     cl = repo.changelog
     torev = cl.nodemap.get
-    for ctx in repo.set('(not public()) and (not obsolete())'):
-        rev = ctx.rev()
+    tonode = cl.node
+    for rev in repo.revs('(not public()) and (not obsolete())'):
         # We only evaluate mutable, non-obsolete revision
-        node = ctx.node()
+        node = tonode(rev)
         # (future) A cache of predecessors may worth if split is very common
         for pnode in obsutil.allpredecessors(repo.obsstore, [node],
                                    ignoreflags=bumpedfix):