Merge branch 'improve-project-mirror-api-quality' into 'master'

Improve project mirror code quality

See merge request gitlab-org/gitlab!28797

ee/lib/api/project_mirror.rb

@@ -30,10 +30,10 @@ module API
       def authenticate_with_webhook_token!
         return not_found! unless project
 
-        unless valid_github_signature?
+        return if valid_github_signature?
+
         render_invalid_github_signature!
       end
-      end
 
       def try_authenticate_with_webhook_token!
         if github_webhook_signature