checkheads: use 'nodemap.get' to convert nodes to revs

We are about to call 'torev' on node that might be locally missing. In this
case, 'nodemap.revs' will return None (something valid in our usecase) while
'changelog.rev' would raise an exception.

We make this change in a distinct changeset to show it does not impact the
tests.

mercurial/discovery.py

@@ -439,7 +439,7 b' def _postprocessobsolete(pushop, futurec'
     repo = pushop.repo
     unfi = repo.unfiltered()
     tonode = unfi.changelog.node
-    torev = unfi.changelog.rev
+    torev = unfi.changelog.nodemap.get
     public = phases.public
     getphase = unfi._phasecache.phase
     ispublic = (lambda r: getphase(unfi, r) == public)