[MERGE] Implement a --diff-command option for bzr diff.

[Nicholas Allen]

>
> The only other bit for integrating difftools instead, is that it can
> handle lots of programs that don't take arguments in the same way. Some
> take FROM TO, others TO FROM, as I understand it also understands 3-way
> diffing, and when you have a command that can only diff 1 file, but you
> give it the whole directory, it can do a recursive diff on the whole tree.
>   
I think it would be nice to have the option to run a diff command 
though. You can solve those problems by using a command pattern (eg 
%FROM, %TO would get substituted with the from and to file names).

So you could do: diff --diff-command "beyondcompare.exe %TO %FROM"

This would be more flexible than diff tools as this needs code for each 
kind of viewer (which is also nice). So I just think it would be great 
to have in built support for many diff viewers (like difftools does) as 
well as totally flexible commands like above.

Cheers,

Nick