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

[Aaron Bentley]

Aaron Bentley has voted resubmit.
Status is now: Resubmit
Comment:
If you're going to introduce external_diff_command, you should deprecate 
external_diff_options.  We don't need both.

I also like --using better than --diff-command.

For details, see: 
http://bundlebuggy.aaronbentley.com/request/%3Cvpq1wdpugsr.fsf%40bauges.imag.fr%3E