index: drop support for negative indexes into the index I want index[i] to work for any valid revnum including -1 (as it already does), and I also want len(index) to be the number of revisions in the index (not counting the null revision), so it cannot also support negative revision numbers other than -1 for nullid. I didn't bother removing support for it from revlog v0. Differential Revision: https://phab.mercurial-scm.org/D4019