…thout prompting

If the repo has multiple remotes, but only one of them is on Github (the others
might for example point to a self-hosted Critic server or something like that),
lazygit would still present a menu to choose the remote for pull requests, but
it would contain only that single entry. That's pointless, pick it automatically
without prompting.

We add some tests while we're at it; these wouldn't have caught the problem,
because they only test getGithubBaseRemote which already takes the filtered
github remotes. It's still better than not having any tests; the real issue
could only have been caught with an integration test, which we don't bother
adding.