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
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