Pass FetchOptions for cloning

[l4l]

So that it's possible to use for private registries with ssh-auth.

[l4l]

I've tested it manually but perhaps it would be nice to add the automatic test like this:

let fo = {
    let mut fo = git2::FetchOptions::new();
    fo.remote_callbacks({
        let mut callbacks = git2::RemoteCallbacks::new();
        callbacks.credentials(|_url, username_from_url, _allowed_types| {
            git2::Cred::ssh_key_from_memory(
                username_from_url.unwrap(),
                None,
                std::env::var("PRIVATE_KEY").unwrap(),
                None,
            )
        });
        callbacks
    });
    fo
};
Index::from_path_or_cloned_with_extra_options(
    "index",
    CloneOptions {
        repository_url: "[email protected]:private-index/goes-here.git".into(),
    },
    Some(fo),
).unwrap();

But it firstly needs to:

• create a private repo with crates index;
• setup key at CI as PRIVATE_KEY.

[Byron]

Thanks a lot! I added the example above as doc-test which doesn't run, that way it's not entirely lost.
Given the simplicity of the change I also feel comfortable not having a test for this.

Some time in the future it should be possible to swap in gitoxide to do all git-related operations, which is when tests should be easier to manufacture tests like these.