Also define a handful of utility methods:
- `New(owner, repo)`
- `FullName`: the name slash owner pair
- `FromFullName`: parse the name slash owner pair
- `FromURL`: parse a GitHub.com URL
- `IsSame(r1, r2)`: compare two repositories
- Check for updates even if `~/.config/gh` does not exist. In this case,
the API call is unauthenticated.
- Avoid having the update notifier ever triggering the OAuth flow.