Add meaningful repr for cache class

[maxfischer2781]

The ConfigCache class is stringified in at least one instance for humans:

condor-git-config/condor_git_config.py

Line 133 in ca7507d

LOGGER.critical("cache %r corrupted by other hook: %r", self, meta_data)

Since it has no __repr__, this just shows the meaningless <ConfigCache object …> default. Instead, it should have a meaningful repr like ConfigCache(git_uri, branch, cache_path, max_age).