tests/test-empty-manifest-index.t

dirstate-item: make sure we load `mtime-second-ambiguous` from disk

Now that we support the associated logic, we can safely load it from it. It is
no longer necessary to ignore the stored mtime when the flag is encountered.

Differential Revision: https://phab.mercurial-scm.org/D11846

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]