Comment 3 for bug 491711

It hasn't landed yet. I believe the last state was that you could have a custom merger when there was divergence, but not when there was only a change on one side.
Oh, and we were trying to figure out what the appropriate api should look like. (If we were going to allow the transform state to be changed by the hook, then it seemed all the state should have been done in the hook.)

And I think Andrew wanted to see if he could refactor so that even the standard code path went via the hook, to make sure that it was 'powerful enough' sort of thing.