locarepo: also fastpath `nullid` lookup in __getitem__

We already use that fastpath for `"null"` and `nullrev`, using it for `nullid`
is similar.

Differential Revision: https://phab.mercurial-scm.org/D7486

mercurial/localrepo.py

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