Git/SVN sidebar widget / block

Show recent commits and messages from git or SVN repositories in the sidebar:

![image](https://user-images.githubusercontent.com/5225190/51444297-bf128680-1ced-11e9-8bce-fcec57d62a79.png)

Wait till WordPress [includes block support in the sidebar](https://make.wordpress.org/core/2018/12/08/gutenberg-phase-2/).