[patch] remove default diff prefix; support -p0 and -p1

[Wayne Davison]

You missed a couple places where --diff-prefix should have changed to
just --prefix:

> -    # TODO: Allow diff across branches.
> +        bzr diff --diff-prefix old/:new/

[...]

> +            if not ':' in prefix:
> +                 raise BzrError("--diff-prefix expects two values separated by a colon")
> +            old_label, new_label = prefix.split(":")

..wayne..