allow referencing existing objects when part of the conflict set

[mpareja]

Issue automerge/automerge#526 demonstrated that it was not possible to resolve a conflict by choosing one of the conflicting objects or arrays. The following change allows setting a nested object to be an existing object reference so long as the referenced object id is currently listed as one of the conflicts for the key.

[mpareja]

This falls down when the array has nested objects.

A makeList operation is added and createNestedObjects is called recursively. The updated and cache maps haven't yet learned about the newly (?) added node.

it('should allow referencing an existing list of objects that is part of the conflict set', () => {
  s2 = Automerge.clone(s1)
  s1 = Automerge.change(s1, doc => doc.list = [{ field: 'a' }])
  s2 = Automerge.change(s2, doc => doc.list = [{ field: 'b' }])

  const result = Automerge.merge(s1, s2)
  const conflicts = Automerge.getConflicts(result, 'list')

  // Should not throw "Cannot create a reference to an existing document object"
  Automerge.change(result, doc => {
    doc.list = Object.values(conflicts)[0]
  })
})