Fix for diffing using iterable_compare_func with nested objects.

[dtorres-sf]

@seperman we have been using deep diff with the iterable_compare_func I helped add a little while back and we ran into an issue with nested objects. If you had an iterable item that moved, and then it had a nested iterable item that got objects added - it was incorrectly merging those objects. So something like:

        t1 = {
            "TestTable": [
                {
                    "id": "022fb580-800e-11ea-a361-39b3dada34b5",
                    "name": "Max",
                    "NestedTable": [
                        {
                            "id": "022fb580-800e-11ea-a361-39b3dada34a6",
                            "NestedField": "Test Field"
                        }
                    ]
                },
                {
                    "id": "022fb580-800e-11ea-a361-12354656532",
                    "name": "Bob",
                    "NestedTable": [
                        {
                            "id": "022fb580-800e-11ea-a361-39b3dada34c7",
                            "NestedField": "Test Field 2"
                        },
                    ]
                },
            ]
        }

        t2 = {"TestTable": [
            {
                "id": "022fb580-800e-11ea-a361-12354656532",
                "name": "Bob (Changed Name)",
                "NestedTable": [
                    {
                        "id": "022fb580-800e-11ea-a361-39b3dada34c7",
                        "NestedField": "Test Field 2 (Changed Nested Field)"
                    },
                    {
                        "id": "new id",
                        "NestedField": "Test Field 3"
                    },
                    {
                        "id": "newer id",
                        "NestedField": "Test Field 4"
                    },
                ]
            },
            {
                "id": "adding_some_random_id",
                "name": "New Name",
                "NestedTable": [
                    {
                        "id": "random_nested_id_added",
                        "NestedField": "New Nested Field"
                    },
                    {
                        "id": "random_nested_id_added2",
                        "NestedField": "New Nested Field2"
                    },
                    {
                        "id": "random_nested_id_added3",
                        "NestedField": "New Nested Field43"
                    },
                ]
            }
        ]}

Merged the items in the t2 nested tables like this rather than splitting it across the item that was moved.

{
  "TestTable": [
    {
      "NestedTable": [
        {
          "id": "random_nested_id_added",
          "NestedField": "New Nested Field"
        },
        {
          "id": "new id",
          "NestedField": "Test Field 3"
        },
        {
          "id": "newer id",
          "NestedField": "Test Field 4"
        }
      ],
      "id": "adding_some_random_id",
      "name": "New Name"
    }
  ]
}

You can see these objects in the new test that was failing without these changes.

I have put comments on the two main changes describing them a bit more.