Factor issue and merge request services

app/services/issuable_base_service.rb

+class IssuableBaseService < BaseService
+  private
+
+  def create_assignee_note(issuable)
+    Note.create_assignee_change_note(
+      issuable, issuable.project, current_user, issuable.assignee)
+  end
+
+  def create_milestone_note(issuable)
+    Note.create_milestone_change_note(
+      issuable, issuable.project, current_user, issuable.milestone)
+  end
+end

app/services/issues/base_service.rb

 module Issues
-  class BaseService < ::BaseService
+  class BaseService < ::IssuableBaseService
 
     private
 
-    def create_assignee_note(issue)
-      Note.create_assignee_change_note(issue, issue.project, current_user, issue.assignee)
-    end
-
     def execute_hooks(issue, action = 'open')
       issue_data = issue.to_hook_data
       issue_url = Gitlab::UrlBuilder.new(:issue).build(issue.id)
       issue_data[:object_attributes].merge!(url: issue_url, action: action)
       issue.project.execute_hooks(issue_data, :issue_hooks)
     end
-
-    def create_milestone_note(issue)
-      Note.create_milestone_change_note(issue, issue.project, current_user, issue.milestone)
-    end
   end
 end

app/services/merge_requests/base_service.rb

 module MergeRequests
-  class BaseService < ::BaseService
-
-    private
-
-    def create_assignee_note(merge_request)
-      Note.create_assignee_change_note(merge_request, merge_request.project, current_user, merge_request.assignee)
-    end
+  class BaseService < ::IssuableBaseService
 
     def create_note(merge_request)
       Note.create_status_change_note(merge_request, merge_request.target_project, current_user, merge_request.state, nil)
@@ -16,9 +10,5 @@ module MergeRequests
         merge_request.project.execute_hooks(merge_request.to_hook_data, :merge_request_hooks)
       end
     end
-
-    def create_milestone_note(merge_request)
-      Note.create_milestone_change_note(merge_request, merge_request.project, current_user, merge_request.milestone)
-    end
   end
 end