index: return False for "len(index) in index"
Since we no longer accept index[len(index)], we should clearly make
"len(index) in index" return False. This should have been part of
a3dacabd476b (index: don't allow index[len(index)] to mean nullid,
2018-07-20)
Differential Revision: https://phab.mercurial-scm.org/D4104