Issue fetching submodules with credentials callback

Summary

Initialising and updating submodules fails when trying to authenticate via SSH key using the credentials callback. From the error message it seems as though setting the callback to pass to libgit2 doesn't actually get passed over? Either that or this is a libgit2 issue.

Details

Setting a callback with an SSH key credential for cloning the repo works correctly. But after cloning the repo and trying to initialise plus update the submodules, with a credential calllback identical to the one for cloning the repo, the following error occurs:

Error { code: -16, klass: 34, message: "remote authentication required but no callback set" }

Code snippet example to replicate issue:

let state = RefCell::new(State::default());

    let mut cb = RemoteCallbacks::new();
    cb.transfer_progress(|stats| {
        let mut state = state.borrow_mut();
        state.progress = Some(stats.to_owned());
        state.display();
        true
    });

    cb.credentials(|_url, username_from_url, _allowed| {
        Cred::ssh_key(
            username_from_url.unwrap_or("git"),
            None,
            ssh_key_path, // Relevant ssh key located in .ssh directory
            None,
        )
    });

    let mut co = CheckoutBuilder::new();
    co.progress(|path, cur, total| {
        let mut state = state.borrow_mut();
        state.path = path.map(Path::to_path_buf);
        state.current = cur;
        state.total = total;
        state.display();
    });

    let mut fo = FetchOptions::new();
    fo.remote_callbacks(cb);

    let repo = RepoBuilder::new()
        .fetch_options(fo)
        .with_checkout(co)
        .branch("develop")
        .clone(
            repo_url,
            Path::new("/tmp/test-git2/"),
        )?;

    for mut submodule in repo.submodules()? {
        let mut update_opts = SubmoduleUpdateOptions::new();
        let mut cb = RemoteCallbacks::new();
        cb.credentials(|_url, username_from_url, _allowed| {
            Cred::ssh_key(
                username_from_url.unwrap_or("git"),
                None,
                ssh_key_path,
                None,
            )
        });
        let mut fo = FetchOptions::new();
        fo.remote_callbacks(cb);

        update_opts.fetch(fo);

        submodule
            .update(true, Some(&mut update_opts))
            .map_err(|err| color_eyre::eyre::eyre!("Failed to update submodule: {:?}", err))?;
    }

Start with the reproduced flow using RepoBuilder::clone, RemoteCallbacks::credentials, SubmoduleUpdateOptions::new, update_opts.fetch, and submodule.update. Compare how the callback is passed during cloning with how submodule updates perform their fetch, then confirm the SSH-key callback is honored and the submodule update completes without the “no callback set” error.