Function process_files_for_fragments 

pub fn process_files_for_fragments(
    files: &[PathBuf],
    root_dir: &Path,
    preferred_revs: &[String],
    seen_frag_ids: &mut FxHashSet<FragmentId>,
    batch_reader: Option<&mut CatFileBatch>,
    is_changed: bool,
) -> Vec<Fragment>