Ref T13124. See PHI531. When a revision is updated, builds against the older diff tend to stop being relevant. Add an option to abort outstanding older builds automatically.
At least for now, I'm adding this as a build step instead of some kind of special checkbox. An alternate implementation would be some kind of "Edit Options" action on plans with a checkbox like [X] When this build starts, abort older builds.
I think adding it as a build step is a bit simpler, and likely to lead to greater consistency and flexibility down the road, make it easier to add options, etc., and since we don't really have any other current use cases for "a bunch of checkboxes". This might change eventually if we add a bunch of checkboxes for some other reason.
The actual step activates before the build queues, so it doesn't need to wait in queue before it can actually act. T13088 discusses some plans here if this sticks.