Forking through GitHub's API is asynchronous so there is really no way
of knowning when you can access git objects without polling.
Alternatively, we can establish the state of the fork with its remote
tracking branches and tags by fetching from the "origin" remote and then
changing the remote url to the fork.
Enterprise installations don't have a separate subdomain for API
requests such as "api.github.com", but their API endpoints start with
"/api/v3" path.
Fixes#201