revlog: remove some knowledge of sentinel nullid in index
I think the "-2" to mean "last position in index, not counting the
null revision at the end" is an implementation detail of the index
that we should avoid spreading knowledge of. I hope we can even remove
support for index[-2].
Differential Revision: https://phab.mercurial-scm.org/D4016
% creating repo
% add and commit
C test.py
% change
M test.py