Expose diff_refs on MergeRequestType

https://gitlab.com/gitlab-org/gitlab-ee/issues/9489

app/graphql/types/diff_refs_type.rb

+# frozen_string_literal: true
+
+module Types
+  # rubocop: disable Graphql/AuthorizeTypes
+  # Types that use DiffRefsType should have their own authorization
+  class DiffRefsType < BaseObject
+    graphql_name 'DiffRefs'
+
+    field :head_sha, GraphQL::STRING_TYPE, null: false, description: 'The sha of the head at the time the comment was made'
+    field :base_sha, GraphQL::STRING_TYPE, null: false, description: 'The merge base of the branch the comment was made on'
+    field :start_sha, GraphQL::STRING_TYPE, null: false, description: 'The sha of the branch being compared against'
+  end
+  # rubocop: enable Graphql/AuthorizeTypes
+end

app/graphql/types/merge_request_type.rb

@@ -23,6 +23,7 @@ module Types
     field :updated_at, Types::TimeType, null: false
     field :source_project, Types::ProjectType, null: true
     field :target_project, Types::ProjectType, null: false
+    field :diff_refs, Types::DiffRefsType, null: true
     # Alias for target_project
     field :project, Types::ProjectType, null: false
     field :project_id, GraphQL::INT_TYPE, null: false, method: :target_project_id

app/graphql/types/notes/diff_position_base_input_type.rb

@@ -5,11 +5,11 @@ module Types
     # rubocop: disable Graphql/AuthorizeTypes
     class DiffPositionBaseInputType < BaseInputObject
       argument :head_sha, GraphQL::STRING_TYPE, required: true,
-               description: copy_field_description(Types::Notes::DiffPositionType, :head_sha)
+               description: copy_field_description(Types::DiffRefsType, :head_sha)
       argument :base_sha,  GraphQL::STRING_TYPE, required: false,
-               description: copy_field_description(Types::Notes::DiffPositionType, :base_sha)
+               description: copy_field_description(Types::DiffRefsType, :base_sha)
       argument :start_sha, GraphQL::STRING_TYPE, required: true,
-               description: copy_field_description(Types::Notes::DiffPositionType, :start_sha)
+               description: copy_field_description(Types::DiffRefsType, :start_sha)
 
       argument :paths,
                Types::DiffPathsInputType,

app/graphql/types/notes/diff_position_type.rb

@@ -7,12 +7,7 @@ module Types
     class DiffPositionType < BaseObject
       graphql_name 'DiffPosition'
 
-      field :head_sha, GraphQL::STRING_TYPE, null: false,
-            description: "The sha of the head at the time the comment was made"
-      field :base_sha,  GraphQL::STRING_TYPE, null: true,
-            description: "The merge base of the branch the comment was made on"
-      field :start_sha, GraphQL::STRING_TYPE, null: false,
-            description: "The sha of the branch being compared against"
+      field :diff_refs, Types::DiffRefsType, null: false
 
       field :file_path, GraphQL::STRING_TYPE, null: false,
             description: "The path of the file that was changed"

spec/graphql/types/diff_refs_type_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe GitlabSchema.types['DiffRefs'] do
+  it { expect(described_class.graphql_name).to eq('DiffRefs') }
+
+  it { expect(described_class).to have_graphql_fields(:base_sha, :head_sha, :start_sha) }
+end

spec/graphql/types/merge_request_type_spec.rb

@@ -13,7 +13,7 @@ describe GitlabSchema.types['MergeRequest'] do
       description_html state created_at updated_at source_project target_project
       project project_id source_project_id target_project_id source_branch
       target_branch work_in_progress merge_when_pipeline_succeeds diff_head_sha
-      merge_commit_sha user_notes_count should_remove_source_branch
+      merge_commit_sha user_notes_count should_remove_source_branch diff_refs
       force_remove_source_branch merge_status in_progress_merge_commit_sha
       merge_error allow_collaboration should_be_rebased rebase_commit_sha
       rebase_in_progress merge_commit_message default_merge_commit_message

spec/graphql/types/notes/diff_position_type_spec.rb

@@ -3,7 +3,7 @@ require 'spec_helper'
 
 describe GitlabSchema.types['DiffPosition'] do
   it 'exposes the expected fields' do
-    expected_fields = [:head_sha, :base_sha, :start_sha, :file_path, :old_path,
+    expected_fields = [:diff_refs, :file_path, :old_path,
                        :new_path, :position_type, :old_line, :new_line, :x, :y,
                        :width, :height]