Implement `cache:untracked`

According to Gitlab [documentation](https://docs.gitlab.com/ci/yaml/#cacheuntracked) , `untracked: true` caches files that are: * Ignored due to [`.gitignore` configuration](https://git-scm.com/docs/gitignore). * Created, but not added to the checkout with [git add](https://git-scm.com/docs/git-add). This is an additional set of files, along with those from `cache:paths`, to add to the cache. **Describe the solution you'd like** Untracked files should be added to the cache. **Describe alternatives you've considered** A warning about `cache:untracked` being ignored in the meanwhile.