Make use of "raw_diff_files" for diffs metadata serializer
The /diffs_metadata
endpoint doesn't need to read the cache for highlighted diff lines, we're able to have some gains for https://gitlab.com/gitlab-org/gitlab/blob/62bbffb4a48bdccea79689536b682bbd9caaa406/app/serializers/diffs_metadata_entity.rb#L5 to use the introduced method at !21446 (merged).
Edited by Oswaldo Ferreira