Perhaps something like
  be-handle-mail --notify-since <revision-id>
to tell subscribers about changes since the specified revision.

This would duplicate mail to P in our first example above, but that's
not too annoying, and P might _want_ to know what R had merged from Q.

On the other hand it would be annoying if 10 other repos merged Q and
ran the notification.

We could make the subscription something like
  subscribe BUG-ID HOST-LIST
e.g.
  subscribe 1234 bugseverywhere.org,fancy_branch.com
  subscribe abcd *
To allow users to whitelist hosts they want updates from.