[Part 3] 5276 Update file mirror and diff modules

What does this MR do?

CE Port => https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/29360

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?

Conformity

• Changelog entry
• 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

Links

• Part of #5276 (closed)