Merge branch 'pb/ort-make-submodule-conflict-message-an-advice'

When a merge conflicted at a submodule, merge-ort backend used to
unconditionally give a lengthy message to suggest how to resolve
it.  Now the message can be squelched as an advice message.

* pb/ort-make-submodule-conflict-message-an-advice:
  merge-ort: turn submodule conflict suggestions into an advice