Fixes diff discussions not being fully removed
What does this MR do?
This fixes a bug where the comment row on none changed lines would also be visible even after deleting the discussion on that line.
It also fixes a very rare edge case where discussions could get assigned to diff lines twice.
What are the relevant issue numbers?
Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/53317
Does this MR meet the acceptance criteria?
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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.