much improved zsh completion script

[Steve Borho]

I've backed off the usage of 'bzr unknowns, etc' in the completion rules and 
replaced them with zsh's built-in file completer.  So the add and commit 
completion functions will now do reasonable things when you use them.

I think this version is safe to be used for general use (and is a big 
improvement over the _bzr on the tip).

http://www.borho.org/~steve/bzr-zsh

Cheers.

--
Steve