issue270_incremental_diff

* For the first added to the system file there is only added entry.
* We store only non empty diffs - if there is no added or removed changes then appropriate file do not stored ([1](https://github.com/bbfsdev/bbfs/blob/issue270_incremental_diff/lib/content_server/content_data_db.rb#L70), [2](https://github.com/bbfsdev/bbfs/blob/issue270_incremental_diff/lib/content_server/content_data_db.rb#L72))