zsh: teach `git hub show` to assume current branch
zsh: teach `git hub show` to assume current branch

Previously required an explicit argument. Now, when no argument is
provided we try to infer the current PR based on the checked-out branch.

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