Guarantee the earliest key

lib/gitlab/background_migration/update_authorized_keys_file_since.rb

@@ -20,7 +20,7 @@ module Gitlab
       end
 
       def add_keys_since(cutoff_datetime)
-        start_key = Key.select(:id).where("created_at >= ?", cutoff_datetime).take
+        start_key = Key.select(:id).where("created_at >= ?", cutoff_datetime).order('id ASC').take
         if start_key
           batch_add_keys_in_db_starting_from(start_key.id)
         end

spec/lib/gitlab/background_migration/update_authorized_keys_file_since_spec.rb

@@ -49,6 +49,7 @@ describe Gitlab::BackgroundMigration::UpdateAuthorizedKeysFileSince do
       before do
         Timecop.travel 1.day.from_now
         @key = create(:key)
+        create(:key) # other key
       end
 
       it 'calls batch_add_keys_in_db_starting_from with the start key ID' do