As part of offering GHE compatibility #273, we need to make some UI decisions on how to onboard users, i.e. authenticate them against their instance and detect when a local repository is a clone from a GHE host as opposed to from github.com.

1. When in the context of a git repository, we might detect that the main git remote does not point to a github.com host, but to example.org, for example.

   • How should we assume if example.org is a GHE host that we should try authenticating against?
   • How should the authentication flow look like? E.g. Press Enter to open https://example.org/authorize in your browser
   • If there are several git remotes of which some point to github.com and others to a GHE host, which one should we default to?

2. When not in the context of a git repository, how should users instruct commands such as repo create, repo clone owner/repo, repo fork owner/repo, and commands such as issue view 123 -R owner/repo to operate on their GHE instance instead of github.com?

   • In hub, users may set GITHUB_HOST=example.org in their environment before invoking a hub command, and that solution is fine for programmatic use, but not ideal for casual use.

• Also in hub, whenever a user has previously authenticated against their GHE host, commands like hub clone repo prompt with Select host: and github.com, example.org as options. This works, but I find the interactive prompt sometimes tiresome considering I'm cloning from github.com 99% of the time.

/cc @ampinsk @billygriffin