Merge branch 'dm-downtime-check-only-added-files' into 'master'

Only check new migrations in 'rake downtime_check'

See merge request !9785

lib/tasks/gitlab/db.rake

@@ -62,7 +62,7 @@ namespace :gitlab do
 
       ref = Shellwords.escape(args[:ref])
 
-      migrations = `git diff #{ref}.. --name-only -- db/migrate`.lines
+      migrations = `git diff #{ref}.. --diff-filter=A --name-only -- db/migrate`.lines
         .map { |file| Rails.root.join(file.strip).to_s }
         .select { |file| File.file?(file) }