Fixed multiple discussions getting added to diff lines

What does this MR do?

Fixes multiple diff discussions getting added to diff lines. This is most obvious with batch comments, when we post a review we refetch all discussions and apply them to their lines which results in multiple discussions.

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/8195

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• 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