index: drop support for nullid at position len(index) in index_node...
index: drop support for nullid at position len(index) in index_node
I think no callers exist since at least
a3dacabd476b (index: don't
allow index[len(index)] to mean nullid, 2018-07-20).
Differential Revision:
https://phab.mercurial-scm.org/D4105
