Database testing pipeline fails if a migration is deleted

If a migration file is deleted then http://gitlab.com/gitlab-org/database-team/gitlab-com-database-testing/blob/master/docker/gitlab/list_migrations.rb#L29-29 will show it as a file to read and parse the migration name, and the lookup will fail. This makes the pipeline fail.

To fix, change the git invocation to only list added files.

This happened in testing for gitlab-org/gitlab!80562 (merged)