parsers: adjust pure-python version to mimic a3dacabd476b

This was caught by the tests, to my surprise. I'll also follow up with
a test of the index[-1] behavior so we can be sure that remains
consistent, as I think that currently has no coverage.

Differential Revision: https://phab.mercurial-scm.org/D4183

mercurial/pure/parsers.py

@@ -52,7 +52,7 b' class BaseIndexObject(object):'
         return i
 
     def __getitem__(self, i):
-        if i == -1 or i == len(self):
+        if i == -1:
             return (0, 0, 0, -1, -1, -1, -1, nullid)
         i = self._fix_index(i)
         if i >= self._lgt: