tests-pure: fixing test-parseindex2 This is a followup to D7603 (49fa0b31ee1d) which broke the tests for pure Python implementation. There are two divergences between pure and C implementations: - the pure implementation would accept only -1 as slice end, whereas C accepts both -1 and len(index) - in pure Python, `headrevs` is provided by revlog.py, not by the index. Differential Revision: https://phab.mercurial-scm.org/D7707