Find a way to merge hub and code credential storage

[begedin]

Currently, the hub command line tool which code uses internally in some cases has its own credential storage under ~/.config/hub.

The code gem itself stores its own set of credentials (just the oauth token) inside the global git configuration file (~/.gitconfig).

The problem is the publish command. It first uses the API to check if there are open PRs for the current branch. Then it uses hub to create a PR.

On a fresh system, the result of that would be that the user is asked for their credentials twice, first once by the API, then once by hub.

Ideally, we would have one set of stored credentials.

If we intend to stick with using hub internally, we should be accessing hub credentials. They are stored in plaintext and easily readable.

If we intend to not rely on hub at some point, then we should somehow provide hub with the code-stored set of credentials, if that's possible.

I think the first choice (assume we will keep using hub) is prefered.