[PLUGIN] bzr_difftools update

[Stephen Ward]

Hi,

I've updated the bzr_difftools plugin with the following changes:

Bug Fixes:

* generate a better error message if a tool is not found
* fix return value to match 'diff' (most external tools don't)

Internal Refactoring:

* moved temporary directory handling to a NamedTemporaryDir class
* reworked the DiffTool class into a small hierarchy:

  ExternalTool
    DiffTool
      TreeDiffTool
      ListDiffTool

  with an eye toward adding a MergeTool class in future.


The branch and tarballs are at the usual location (see the PluginRegistry).

-- Steve