Trivial format strings adjustments.
* test_pack_repository.py:
(load_tests): Not before 1.13 for development4 formats.
* repository.py:
(RepositoryFormatPackDevelopment4,
RepositoryFormatPackDevelopment4Subtree): Not before 1.13.
* pack_repo.py:
(RepositoryFormatPackDevelopment4.get_format_string,
RepositoryFormatPackDevelopment4Subtree.get_format_string): Not
before 1.13.