index: use `index.get_rev` in `obsolete._computephasedivergentset`

Differential Revision: https://phab.mercurial-scm.org/D7336

mercurial/obsolete.py

@@ -980,7 +980,7 b' def _computephasedivergentset(repo):'
     phase = repo._phasecache.phase  # would be faster to grab the full list
     public = phases.public
     cl = repo.changelog
-    torev = cl.nodemap.get
+    torev = cl.index.get_rev
     tonode = cl.node
     obsstore = repo.obsstore
     for rev in repo.revs(b'(not public()) and (not obsolete())'):