[PATCH 0 of 2] Better support for diffing changesets and 3-way extdiff.

[Sune Foldager]

This improves my previous patch set in a few ways:

- Fixed a few bugs with would lead to unexpected behavior and allow you to
  select --switch-parent even when there was only one.
- Removed changes to cmdutil.py, instead handling things directly in the command.
- Implemented Mads Kiilerich's 3-way extdiff patch in hgext/extdiff.py.
- Removed --switch-parent on extdiff as a result.

-- Sune.