Function codealong::with_authentication

pub fn with_authentication<T, F>(
    url: &str, 
    cfg: &Config, 
    f: F
) -> Result<T, Error> where
    F: FnMut(&mut Credentials) -> Result<T, Error>, 

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 as possible based on that:

• Prioritize SSH keys from the local ssh agent as they're likely the most reliable. The username here is prioritized from the credential callback, then from whatever is configured in git itself, and finally we fall back to the generic user of git.

• If a username/password is allowed, then we fallback to git2-rs's implementation of the credential helper. This is what is configured with credential.helper in git, and is the interface for the OSX keychain, for example.

• After the above two have failed, we just kinda grapple attempting to return something.

If any form of authentication fails, libgit2 will repeatedly ask us for credentials until we give it a reason to not do so. To ensure we don't just sit here looping forever we keep track of authentications we've attempted and we don't try the same ones again.