[Part 3] 5276 Update file mirror and diff modules
What does this MR do?
This MR is part of the larger https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/12239.
This MR makes the necessary updates for the file mirror and create diff modules to be fully functional.
Does this MR meet the acceptance criteria?
- Documentation created/updated or follow-up review issue created
- Code review guidelines
- Merge request performance guidelines
- Style guides
- Database guides
- Separation of EE specific content
Performance and testing
- Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
- Tested in all supported browsers
- Part of #5276 (closed)