Add a composite primary key for diff files

What does this MR do?

Configures a composite primary key for merge request diff files

We don't actually use this at present, but it allows Rails to reliably update or delete a single row in the merge_request_diff_files table, which seems like a good thing to have for console manipulations and future functionality.

I added this during a WIP stage of https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24276 - I may not need it for that functionality in the end, but I thought I'd split it out and get it merged separately either way.

What are the relevant issue numbers?

Related to #52568 (closed)

Does this MR meet the acceptance criteria?

• [-] Changelog entry added, if necessary
• [-] Documentation created/updated via this MR
• [-] Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• [-] Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• [-] Conforms to the style guides
• Conforms to the database guides
• [-] Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• [-] Security reports checked/validated by reviewer