Function diff_tree_to_worktree_with_git_dir 

pub fn diff_tree_to_worktree_with_git_dir(
    odb: &Odb,
    tree_oid: Option<&ObjectId>,
    work_tree: &Path,
    repository_git_dir: &Path,
    index: &Index,
    config: Option<&ConfigSet>,
) -> Result<Vec<DiffEntry>>

Like diff_tree_to_worktree, but loads config from repository_git_dir (linked worktrees / gitfiles).

Pass config when the caller already holds a repository config snapshot.