Use nullrev+count instead of count-1 in a newly introduced expression.

Before that repo.changelog.tip() was used, but this yields a node.

Of course many more places should be changed like this, this was just done
as an example.

mercurial/commands.py

@@ -2195,7 +2195,7 b' def tip(ui, repo, **opts):'
 
     Show the tip revision.
     """
-    cmdutil.show_changeset(ui, repo, opts).show(repo.changelog.count()-1)
+    cmdutil.show_changeset(ui, repo, opts).show(nullrev+repo.changelog.count())
 
 def unbundle(ui, repo, fname, **opts):
     """apply a changegroup file