index: handle index[-1] as nullid more explicitly
I find it more intuitive to check if "pos == -1" than to first add the
index length (which includes one extra item for the nullid) and
compare that to "length - 1". However, because test-parseindex2.py
compares the whole index (up to len(index)-1), we need to also
preserve that other check for a little while more. I'll remove it
soon.
Differential Revision: https://phab.mercurial-scm.org/D4018
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