index: use `index.get_rev` in `phases.newheads`

Differential Revision: https://phab.mercurial-scm.org/D7332

mercurial/phases.py

@@ -733,7 +733,7 b' def newheads(repo, heads, roots):'
 
     repo = repo.unfiltered()
     cl = repo.changelog
-    rev = cl.nodemap.get
+    rev = cl.index.get_rev
     if not roots:
         return heads
     if not heads or heads == [nullid]: