Depends on D20181. Depends on D20182. Fixes T3498. Ref T13161. My claim, at least, is that D20181 can be tweaked to be good enough to throw away this "feature" completely.
I think this feature was sort of a mistake, where the ease of access to diff -bw shaped behavior a very long time ago and then the train just ran a long way down the tracks in the same direction.