feat(zsh): add `git hub fetch`
feat(zsh): add `git hub fetch`

To facilitate use cases like wanting to get multiple PRs but not
actually checking them out.

Sometimes people send me a pull and then instead of updating it they
send another. With this command I can fetch them both and run `git diff
pull/{71,76}` on them.

Also, it's an alternative to `git hub update`: I can rerun `git fetch`
to update an existing branch and then do a diff or a `git reset --hard`
etc.

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