tests/test-empty-manifest-index.t

bundlerepo: fix mismatches with repository and revlog classes

Both pytype and PyCharm complained that `write()` and `_write()` in the
bundlephasecache class aren't proper overrides- indeed they seem to be missing
an argument that the base class has.

PyCharm and pytype also complained that the `revlog.revlog` class doesn't have a
`_chunk()` method.  That looks like it was moved from revlog to `_InnerRevlog`
back in e8ad6d8de8b8, and wasn't caught because this module wasn't type checked.
However, I couldn't figure out a syntax with `revlog.revlog._inner._chunk(self, rev)`,
as it complained about passing too many args.  `bundlerevlog._rawtext()` uses
this `super(...)` style to call the super class, so hopefully that works, even
with the wonky dynamic subclassing.  The revlog class needed the `_InnerRevlog`
field typed because it isn't set in the constructor.

Finally, the vfs type hints look broken.  This initially failed with:

    File "/mnt/c/Users/Matt/hg/mercurial/bundlerepo.py", line 65, in __init__: Function readonlyvfs.__init__ was called with the wrong arguments [wrong-arg-types]
             Expected: (self, vfs: mercurial.vfs.vfs)
      Actually passed: (self, vfs: Callable)
    Called from (traceback):
      line 232, in dirlog
      line 214, in __init__

I don't see a raw Callable, but I tried changing some of the vfs args to be typed
as `vfsmod.abstractvfs`, but that class doesn't have `options`, so it failed
elsewhere.  `readonlyvfs` isn't a subclass of `vfs` (it's a subclass of
`abstractvfs`), so I'm not sure how to handle that.  It would be a shame to have
to make a union of vfs subclasses (but not all of them have `options` either).

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]