Merge pull request #62004 from apple/maxd/fix-checkout-cross-repo
update_checkout: re-read config for cross-repo testing
When updating versions of some dependencies, it's important to be able to re-read the checkout config to get the new versions to actually check them out. This not working prevents us from making CI green in cross-repo PRs that update these dependencies.
I also used this as an opportunity to refactor out some common code and add Python docstrings to some functions for easier maintenance.
