Add reviewer information to new_mention_in_merge_request_email

- it also fixes broken assignees_label

app/mailers/previews/notify_preview.rb

@@ -60,6 +60,10 @@ class NotifyPreview < ActionMailer::Preview
     end
   end
 
+  def new_mention_in_merge_request_email
+    Notify.new_mention_in_merge_request_email(user.id, issue.id, user.id).message
+  end
+
   def closed_issue_email
     Notify.closed_issue_email(user.id, issue.id, user.id).message
   end

app/views/notify/new_mention_in_merge_request_email.text.erb

@@ -4,6 +4,7 @@ You have been mentioned in Merge Request <%= @merge_request.to_reference %>
 
 <%= merge_path_description(@merge_request, 'to') %>
 Author:    <%= sanitize_name(@merge_request.author_name) %>
-= assignees_label(@merge_request)
+<%= assignees_label(@merge_request) %>
+<%= reviewers_label(@merge_request) %>
 
 <%= @merge_request.description %>

changelogs/unreleased/300750-add-missing-reviewers-information-to-new_mention_in_merge_request_.yml

+---
+title: Add reviewers detail to new mention in merge request email
+merge_request: 56184
+author:
+type: added

spec/mailers/emails/merge_requests_spec.rb

@@ -24,6 +24,23 @@ RSpec.describe Emails::MergeRequests do
   let(:recipient) { assignee }
   let(:current_user_sanitized) { 'www_example_com' }
 
+  describe '#new_mention_in_merge_request_email' do
+    subject { Notify.new_mention_in_merge_request_email(recipient.id, merge_request.id, current_user.id) }
+
+    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(project_merge_request_path(project, merge_request))
+        is_expected.to have_body_text('You have been mentioned in Merge Request')
+        is_expected.to have_link(merge_request.to_reference, href: project_merge_request_url(merge_request.target_project, merge_request))
+        is_expected.to have_text_part_content(assignee.name)
+        is_expected.to have_text_part_content(reviewer.name)
+        is_expected.to have_html_part_content(assignee.name)
+        is_expected.to have_html_part_content(reviewer.name)
+      end
+    end
+  end
+
   describe '#merge_request_unmergeable_email' do
     subject { Notify.merge_request_unmergeable_email(recipient.id, merge_request.id) }