backout: deprecate/hide support for backing out merges This has never worked usefully: - it can't undo a completely unwanted merge, as it leaves the merge in the DAG - it can't undo a faulty merge as that means doing a merge correctly, not simply reverting to one or the other parent Both of these kinds of merge also require coordinated action among developers to avoid the bad merge continuing to affect future merges, so we should stop pretending that backout is of any help here. As backing out a merge now requires a hidden option, it can't be done by accident, but will continue to 'work' for anyone who's already dependent on --parent for some unknown reason.