Fork fetch illusion

See [#301](https://github.com/github/hub/issues/301#issuecomment-32990717).

Forking is done asynchronously through the GitHub API call so running `git remote add -f ...` can result in a error if the fork hasn't been created yet. 

This pull request uses an alternative to fetching the fork. It insteads fetches from the "origin" and then set the remote url to the fork.