tests/test-empty-manifest-index.t

setup: drop support for Python 3.5

We talked about this on the mailing list [1] and there seemed to be
agreement that Python 3.5 is effectively dead and no longer worth
supporting.

So this commit changes our minimum version requirement to 3.6.2.

[1] https://www.mercurial-scm.org/pipermail/mercurial-devel/2022-February/147885.html

Differential Revision: https://phab.mercurial-scm.org/D12315

Test null revisions (node 0000000000000000000000000000000000000000, aka rev -1)
in various circumstances.

Make an empty repo:

  $ hg init a
  $ cd a

  $ hg files -r 0000000000000000000000000000000000000000
  [1]
  $ hg files -r .
  [1]

Add an empty commit (this makes the changelog refer to a null manifest node):


  $ hg commit -m "init" --config ui.allowemptycommit=true

  $ hg files -r .
  [1]

Strip that empty commit (this makes the changelog file empty, as opposed to missing):

  $ hg --config 'extensions.strip=' strip . > /dev/null

  $ hg files -r .
  [1]