This is the packaged version of Collab, currently limited to containing the History and Toolbar windows, along with supporting classes.
Check this repository out in your {$PROJECT}/Packages/ folder, under the name com.unity.collab-proxy. The classes will be built by Unity.
In order to run the tests, you will need to add this project to the testables key in your manifest.json - once you have done this, the tests will be picked up by the Unity Test Runner window.
You may build this project using msbuild. The commands to do so can be seen under .gitlab-ci.yml.
Gitlab will automatically build your project when you deploy. You can download the resulting artifact, which will be a dll, and place it in your Editor/bin/ folder. Open the package in Unity to generate the meta files, and then you will be able to publish.
We're currently looking into a way to avoid this manual process.