Function with_authentication

Prepare the authentication callbacks for cloning a git repository. The main purpose of this function is to construct the "authentication callback" which is used to clone a repository. This callback will attempt to find the right authentication on the system (without user input) and will guide libgit2 in doing so. The callback is provided `allowed` types of credentials, and we try to do as much a

(url: &str, cfg: &git2::Config, mut f: F)