localrepo: also fastpath `nullrev` in __getitem__

As explained earlier, nullrev will exist in all repository, we do not need any
special checking.

Differential Revision: https://phab.mercurial-scm.org/D7480

mercurial/localrepo.py

@@ -1531,7 +1531,7 b' class localrepository(object):'
             ]
 
         # dealing with some special values
-        if changeid == b'null':
+        if changeid == b'null' or changeid == nullrev:
             return context.changectx(self, nullrev, nullid)
         if changeid == b'tip':
             node = self.changelog.tip()