Fix body of email when commits are pushed to an MR

This was sending the current user, which is the recipient! It should be the user
who pushed the commits.

app/mailers/emails/merge_requests.rb

@@ -15,6 +15,7 @@ module Emails
       setup_merge_request_mail(merge_request_id, recipient_id)
       @new_commits = new_commits
       @existing_commits = existing_commits
+      @updated_by_user = User.find(updated_by_user_id)
 
       mail_answer_thread(@merge_request, merge_request_thread_options(updated_by_user_id, recipient_id, reason))
     end

app/views/notify/push_to_merge_request_email.html.haml

 %h3
-  New commits were pushed to the merge request
+  = @updated_by_user.name
+  pushed new commits to merge request
   = link_to(@merge_request.to_reference, project_merge_request_url(@merge_request.target_project, @merge_request))
-  by #{@current_user.name}
 
 - if @existing_commits.any?
   - count = @existing_commits.size

app/views/notify/push_to_merge_request_email.text.haml

-New commits were pushed to the merge request #{@merge_request.to_reference} by #{@current_user.name}
+#{@updated_by_user.name} pushed new commits to merge request #{@merge_request.to_reference}
 \
 #{url_for(project_merge_request_url(@merge_request.target_project, @merge_request))}
 \

spec/mailers/notify_spec.rb

@@ -389,6 +389,36 @@ describe Notify do
           end
         end
       end
+
+      describe 'that have new commits' do
+        let(:push_user) { create(:user) }
+
+        subject do
+          described_class.push_to_merge_request_email(recipient.id, merge_request.id, push_user.id, new_commits: merge_request.commits)
+        end
+
+        it_behaves_like 'a multiple recipients email'
+        it_behaves_like 'an answer to an existing thread with reply-by-email enabled' do
+          let(:model) { merge_request }
+        end
+        it_behaves_like 'it should show Gmail Actions View Merge request link'
+        it_behaves_like 'an unsubscribeable thread'
+
+        it 'is sent as the push user' do
+          sender = subject.header[:from].addrs[0]
+
+          expect(sender.display_name).to eq(push_user.name)
+          expect(sender.address).to eq(gitlab_sender)
+        end
+
+        it 'has the correct subject and body' do
+          aggregate_failures do
+            is_expected.to have_referable_subject(merge_request, reply: true)
+            is_expected.to have_body_text("#{push_user.name} pushed new commits")
+            is_expected.to have_body_text(project_merge_request_path(project, merge_request))
+          end
+        end
+      end
     end
 
     context 'for issue notes' do