Function get_worktree_file_diff 

pub fn get_worktree_file_diff(
    repo_path: &Path,
    file_path: &str,
    staged: bool,
) -> Vec<DiffLine>

Return the diff for file_path in the working tree.

• staged = true: diff between HEAD and the index (what would be committed).
• staged = false: diff between the index and the working directory (unstaged changes).

Returns an empty Vec on any error.