Buildbot failure emails go to too many recipients

[ryandesign (Ryan Carsten Schmidt)]

Each buildbot failure email is sent not only to the maintainers of the ports affected by this build failure, but also to the recipients of all previous build failure emails.

I've looked at the code​ for a long time and don't understand it. Obviously the list of interested users isn't being cleared at the beginning of each build. But I don't see where in the code anything gets added to the list of interested users. I see that we have a custom PortsMailNotifier class that inherits from MailNotifier, which is "same as original, but calls portMessageFormatter with access to interested_users". But I don't see where portWatcherMessageFormatter ever makes use of the interested_users argument its been passed.