Workflow best practices for migration of GitHub code into internal repo

This looks like a great tool, and would be of use to us on some upcoming projects. Thanks for open sourcing it!

One question I have (I can't seem to find a better forum for Q's) is around best practices for integrating changes from GitHub with an internal authoritative repo.

This is mentioned in the README:

> Importing a change from a non-authoritative repository into the authoritative repository. When a change is made in the non-authoritative repository (for example, a contributor in the public repository), Copybara transforms and moves that change into the appropriate place in the authoritative repository. Any merge conflicts are dealt with in the same way as an out-of-date change within the authoritative repository.

Concretely, say someone opens a PR on your GitHub repo. What does the process look like for integrating that change into the internal repo? Is the change first merged into the GitHub repo then migrated to the internal repo? Or is there some way to do some kind of initial vetting to know that the change is compatible with the internal repo before merging the change in GitHub, then moving? I've often wondered how this is done on some of the PRs I've submitted to Google GitHub projects.

Happy to offer up some documentation in a PR once I grok this concept.

Thanks again for open sourcing the tool!