Note that this patch changes the tracker and not core code.

Also I have a tracker where I can create a "meta" issue.
I can link other issues (group them) into the meta issue.
By updating the meta issue, the same message is sent to all
the grouped issues.

This is an alternate way of merging multiple issues and treating
them as a single issue.

I am not quite sure but this patch seems to break that use case.

I wonder if it would be better to add this as an example on the wiki
and link to this ticket.

I do like providing a ui for linking/unlinking messages
and providing an undo action for a spurious unlink.

Maybe that should go into the classic tracker.