Delete non-latest merge request diff files upon MRs merge
What does this MR do?
Deletes non-latest merge request diff files upon MRs merge.
This MR doesn't cover the background migration which will cleanup diff files from old MRs (ongoing on https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/20013).
Reviewer should double-check
MergeRequest#merge_request_diff is safe to use in this context? I believe it'll always return a record, since we create a new merge request diff before deleting the non-latest.
Does this MR meet the acceptance criteria?
- Changelog entry added, if necessary
- Documentation created/updated
- API support added
- Tests added for this feature/bug
- Conform by the code review guidelines
- Has been reviewed by a Backend maintainer
- Conform by the merge request performance guides
- Conform by the style guides
- If you have multiple commits, please combine them into a few logically organized commits by squashing them
- Internationalization required/considered
End-to-end tests pass (
package-and-qamanual pipeline job)