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