index: don't include nullid in boundary check in pure code
Yet another change that should probably 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/D4333
pager.pager=p0 # $TESTTMP/sysrc:4
ui.editor=e1 # $TESTTMP/userrc:2
pager.pager=p2 # $PAGER
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