So like a branch that automatically rebases on another branch? I think the issue with this is the inevitable conflicts and race-conditions. While nice in theory, often you need manual control to sort things out.
I guess I was imagining a conflict as the result of a race condition. Two concurrent changes are made where if they were ordered, there would be no conflict.
Ideally it should be no different to the current situation; you cannot push to a branch that has been pushed to since your last pull - the difference being that changes to the parent are also considered.
