def init_repo(repo):
    repo = OSTree.Repo.new(Gio.File.new_for_path(repo))
    repo.open()
    return repo

def diff_commits(args):
    repo = init_repo(args.repo)
    a = args.ref_a
    b = args.ref_b

    _, root_a, _ = repo.read_commit(a)
    _, root_b, _ = repo.read_commit(b)

    modified = []
    removed = []
    added = []

    OSTree.diff_dirs(OSTree.DiffFlags.NONE, root_a, root_b, modified, removed, added)

    print(root_a.get_root().get_contents_checksum())
    print(root_b.get_root().get_contents_checksum())
    print(modified, removed, added)