Split brain: Start introducing gbp (split brain) quilt mode

* Replace quilt_could_gbp with new calculation based on three calls to
  quiltify_trees_differ.

* Reworked messages about quilt differences.

* Start to provide the `gbp', `apply' and `dpm' quilt modes.
  Currently we crash with a die if any of these are used.
  Nevertheless, we allow `gbp' and `apply' to be specified.