Existing notes on merge requests aren't displayed on merge (head) version

Support for merge-ref-comments has been introduced at !28623 (merged). The existing notes don't have diff_note_positions generated for them, i.e they aren't going to be displayed on merge-ref-head diff until the mergeability check is run for a merge request.

We need either to migrate data (which may turn up to be a high-load operation) or generate the data on the fly, when merge-ref-head version is requested

Possible solution: !28623 (comment 319336227)

Related issue: #198457 (closed)

Feature flags

return unless Feature.enabled?(:merge_ref_head_comments, @merge_request.target_project, default_enabled: true)
return unless Feature.enabled?(:merge_red_head_comments_position_on_demand, @merge_request.target_project, default_enabled: true)