Diff comments in merge proposals get lost if not explicitly saved

Inline diff comments in MPs have each their own Save/Cancel actions, separate from the main "Save comment" action at the top. This "Save" duplicity might be confusing and cause loss of inline comments, as the user might save the main comment expecting diff comments to be included - but they're dropped without warning. In such case I'd suggest saving inline comments automatically, or at least show a warning about unsaved diff comments.