[bug] [polymorphism] Changes in child schema are ignored when only parent schema is directly referenced

When using AllOf for polymorphism, when a property references a parent schema using ref and a change is done to a child schema which is not referenced directly anywhere in the OAS file, changes done to child schema are ignored in the differ output.

Example schema:

openapi: 3.0.0
servers:
  - url: 'http://petstore.swagger.io/v2'
info:
  description: >-
    This is a sample server Petstore server.  You can find out more about
    Swagger at [http://swagger.io](http://swagger.io) or on [irc.freenode.net,
    #swagger](http://swagger.io/irc/).  For this sample, you can use the api key
    `special-key` to test the authorization filters.
  version: 1.0.0
  title: Swagger Petstore
  termsOfService: 'http://swagger.io/terms/'
  contact:
    email: apiteam@swagger.io
  license:
    name: Apache 2.0
    url: 'http://www.apache.org/licenses/LICENSE-2.0.html'
tags:
  - name: pet
    description: Everything about your Pets
    externalDocs:
      description: Find out more
      url: 'http://swagger.io'
  - name: store
    description: Access to Petstore orders
  - name: user
    description: Operations about user
    externalDocs:
      description: Find out more about our store
      url: 'http://swagger.io'
paths:
  /pet/findByStatus:
    get:
      tags:
        - pet
      summary: Finds Pets by status
      description: Multiple status values can be provided with comma separated strings
      operationId: findPetsByStatus
      parameters:
        - name: status
          in: query
          description: Status values that need to be considered for filter
          required: true
          explode: true
          schema:
            type: array
            items:
              type: string
              enum:
                - available
                - pending
                - sold
              default: available
      responses:
        '200':
          description: successful operation
          content:
            application/json:
              schema:
                type: object
                properties:
                  pets:
                    type: array
                    items:
                      $ref: '#/components/schemas/Pet'
        '400':
          description: Invalid status value
      security:
        - petstore_auth:
            - 'write:pets'
            - 'read:pets'
externalDocs:
  description: Find out more about Swagger
  url: 'http://swagger.io'
components:
  requestBodies:
    Pet:
      content:
        application/json:
          schema:
            $ref: '#/components/schemas/Pet'
      description: Pet object that needs to be added to the store
      required: true
  securitySchemes:
    petstore_auth:
      type: oauth2
      flows:
        implicit:
          authorizationUrl: 'http://petstore.swagger.io/oauth/dialog'
          scopes:
            'write:pets': modify pets in your account
            'read:pets': read your pets
    api_key:
      type: apiKey
      name: api_key
      in: header
  schemas:
    BasePet:
      type: object
      properties:
        pet_color:
          type: string
    Pet:
      allOf:
      - $ref: '#/components/schemas/BasePet'
      type: object
      discriminator:
        propertyName: pet_type
        mapping:
          dog: '#/components/schemas/Dog'
          cat: '#/components/schemas/Cat'
      required:
      - pet_type
      properties:
        pet_type:
          nullable: false
          allOf:
          - type: string
    Cat:
      description: Cat class
      allOf:
      - $ref: '#/components/schemas/Pet'
      type: object
      discriminator:
        propertyName: pet_type
        mapping:
          dog: '#/components/schemas/Cat'
      properties:
        name:
          type: string
        toy:
          $ref: '#/components/schemas/Toy'
    Dog:
      description: Dog class
      allOf:
      - $ref: '#/components/schemas/Pet'
      type: object
      discriminator:
        propertyName: pet_type
        mapping:
          dog: '#/components/schemas/Cat'
      properties:
        bark:
          type: string
    Toy:
      description: Toy class
      type: object
      discriminator:
        propertyName: toy_type
        mapping:
          ball: '#/components/schemas/Ball'
      properties:
        toy_type:
          type: string
        price:
          type: number
    Ball:
      description: Ball class
      allOf:
        - $ref: '#/components/schemas/Toy'
        - type: object
          properties:
            size:
              type: string
      discriminator:
        propertyName: toy_type
        mapping:
          ball: '#/components/schemas/Ball'

If I add new property material to Ball, I would expect to see changes in GET /pet/findByStatus operation, while the actual situation is, that the changes are ignored.

Tested with 2.1.0-SNAPSHOT.

Start by reproducing the reported case with the supplied OpenAPI document and compare the differ output after adding Ball.material. Trace how the GET /pet/findByStatus reference reaches the polymorphic schemas and confirm that changes in indirectly referenced child schemas are included. Done means the output reports the Ball change and a regression test covers this scenario.