Include "Merge-request reopened" emails in the proper email thread

app/mailers/emails/merge_requests.rb

@@ -49,7 +49,8 @@ module Emails
       @updated_by = User.find updated_by_user_id
       @target_url = project_merge_request_url(@project, @merge_request)
       set_reference("merge_request_#{merge_request_id}")
-      mail(from: sender(updated_by_user_id),
+      mail_answer_thread(@merge_request,
+                         from: sender(updated_by_user_id),
                          to: recipient(recipient_id),
                          subject: subject("#{@merge_request.title} (##{@merge_request.iid}) #{@mr_status}"))
     end

spec/mailers/notify_spec.rb

@@ -322,6 +322,35 @@ describe Notify do
           end
         end
 
+        describe 'status changed' do
+          let(:status) { 'reopened' }
+          subject { Notify.merge_request_status_email(recipient.id, merge_request.id, status, current_user) }
+
+          it_behaves_like 'an answer to an existing thread', 'merge_request'
+
+          it 'is sent as the author' do
+            sender = subject.header[:from].addrs[0]
+            sender.display_name.should eq(current_user.name)
+            sender.address.should eq(gitlab_sender)
+          end
+
+          it 'has the correct subject' do
+            should have_subject /#{merge_request.title} \(##{merge_request.iid}\)/i
+          end
+
+          it 'contains the new status' do
+            should have_body_text /#{status}/i
+          end
+
+          it 'contains the user name' do
+            should have_body_text /#{current_user.name}/i
+          end
+
+          it 'contains a link to the merge request' do
+            should have_body_text /#{project_merge_request_path project, merge_request}/
+          end
+        end
+
         describe 'that are merged' do
           subject { Notify.merged_merge_request_email(recipient.id, merge_request.id, merge_author.id) }