Function compare_old_fingerprint

fn compare_old_fingerprint(
    unit: &Unit,
    old_hash_path: &Path,
    new_fingerprint: &Fingerprint,
    mtime_on_use: bool,
    forced: bool,
) -> Option<DirtyReason>

Reads the value from the old fingerprint hash file and compare.

If dirty, it then restores the detailed information from the fingerprint JSON file, and provides an rich dirty reason.