radicle: use `git fetch` over libigt2 in checkout — heartwood

Similarly to pushing using the file:// protocol and libgit2, fetching will be slow – so instead use git fetch directly to perform the checkout.

 #[derive(Error, Debug)]
 pub enum CheckoutError {
     #[error("failed to fetch to working copy")]
     Fetch(#[source] git2::Error),
     Fetch(#[source] std::io::Error),
     #[error("git: {0}")]
     Git(#[from] git2::Error),
     #[error("payload: {0}")]

         &url,
         &url.clone().with_namespace(*remote),
     )?;
     git::fetch(&repo, &REMOTE_NAME).map_err(CheckoutError::Fetch)?;
     let fetchspec = git::Refspec {
         src: git::refspec::pattern!("refs/heads/*"),
         dst: git::Qualified::from(git::lit::refs_remotes(&*REMOTE_NAME))
             .to_pattern(git::refspec::STAR)
             .into_patternstring(),
         force: false,
     };
     let stored = storage.repository(proj)?;
     git::run::<_, _, &str, &str>(
         storage::git::paths::working_copy(&repo).map_err(CheckoutError::Fetch)?,
         [
             "fetch",
             &format!(
                 "{}",
                 stored
                     .path()
                     .canonicalize()
                     .map_err(CheckoutError::Fetch)?
                     .display()
             ),
             &fetchspec.to_string(),
         ],
         [],
     )
     .map_err(CheckoutError::Fetch)?;
     {
         // Setup default branch.