zsh: simplify conditional
zsh: simplify conditional

The logic here may not be totally clear, so perhaps I should add some
comments to this, but even writing the comments is hard. Previous two
commits explain what is going on.

wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent