There's another _bzr completion function available that was in the bzr
repository (in the contrib/zsh subdir).  This one is more complete than
what we have, so I refined it quite a bit, and then checked it in.

Since many of you folks are much better at completion functions than me,
please feel free to give it a look and see if it can't be improved even
more.  I'll attach it to this message to make it easy to review...

..wayne..