[ANN][PLUGIN] extending diff with graphical tools

[Stephen Ward]

Hi,

I've updated the plugin to disable recursive diff on VIM-derived tools, and to 
use the working-tree (instead of a temp directory) in more cases, to 
facilitate editing-in-place.  I also switched the check for empty diffs to 
use bzrlib.deltas.compare_trees() instead of 'diff --recursive --brief', so 
it is about 6X faster at finding out there's nothing to do.

The initialization for VIM-derived tools now looks like:

  register_diff_tool(DiffTool('gvim', diff_options='-f -d', recursive=False))
  register_diff_tool(DiffTool('gvimdiff', diff_options='-f', recursive=False))
  register_diff_tool(DiffTool('vim', diff_options='-d', recursive=False))
  register_diff_tool(DiffTool('vimdiff', recursive=False))

There are probably still improvements to make in the handling of non-recursive 
tools, but the basic mechanism is in place now.

-- Steve