Martin von Zweigbergk <martinvonz@google.com> [Sat, 04 Aug 2018 22:48:25 -0700] rev 38867
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