tests/test-issue5979.t

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

  $ hg init r1
  $ cd r1
  $ hg ci --config ui.allowemptycommit=true -m c0
  $ hg ci --config ui.allowemptycommit=true -m c1
  $ hg ci --config ui.allowemptycommit=true -m c2
  $ hg co -q 0
  $ hg ci --config ui.allowemptycommit=true -m c3
  created new head
  $ hg co -q 3
  $ hg merge --quiet
  $ hg ci --config ui.allowemptycommit=true -m c4

  $ hg log -G -T'{desc}'
  @    c4
  |\
  | o  c3
  | |
  o |  c2
  | |
  o |  c1
  |/
  o  c0
  

  >>> from mercurial import hg
  >>> from mercurial import ui as uimod
  >>> repo = hg.repository(uimod.ui())
  >>> for anc in repo.changelog.ancestors([4], inclusive=True):
  ...   print(anc)
  4
  3
  2
  1
  0