Draft: Improving incoming/outgoing with client heads that have been obsoleted on the server

The problem here is that concurrent work on separate topics on the same named branch is flagged as adding additional heads when the second client pushes to a server that has already obsoleted some of that independent work. It also shows up as extra incoming when checked the other way.

@marmoute , @av6 I suspect you guys known a lot more about exchange and the wire protocol than I do.