[MERGE] get rid of os.spawnvp() usage for external diff

[Alexander Belchenko]

James Henstridge пишет:
> The new changes are in my branch.  I'll post another patch with the
> NamedTemporaryFile stuff changed.  I'd need someone else to test if it
> fixes the problem on Windows though.

Probably I'm the one of active bzr windows user/tester. So you can ping 
me if I can help to test some code.

(I read not all messages in mailing list so times to times I can miss 
something discussion. So don't hesitate to ping me directly).

--
Alexander