Twenty Sixteen, the default theme scheduled to ship with WordPress 4.4, is available for download on GitHub and the WordPress theme directory. According to Tammie Lister, Twenty Sixteen will be developed as if it were a feature plugin and will merge into WordPress core later this year.
As development takes place on GitHub, changes will regularly sync up to the WordPress theme directory. By installing and activating Twenty Sixteen from the theme directory, users can easily update to new versions as they become available.
So far, Twenty Sixteen has 23 issues and 27 pull requests on GitHub. Many of the issues such as, introducing automated Travis CI build testing into Twenty Sixteen, are up for discussion.
Here is what the top half of the Tavern looks like with Twenty Sixteen activated.

Here is what the content section looks like. Notice the block of code that displays instead of an image.

Testers are encouraged to open issues and pull requests on GitHub. If you’re not familiar with how GitHub works, this guide explains how to contribute to the Twenty Sixteen project. Keep in mind that Twenty Sixteen is a work in progress and should not be used in a production environment.
I was very happy to find this out and am excited by the prospect of more projects being developed on GitHub and merged into core :)
I have never used SVN for anything other than updating a plugin on the WordPress.org repo, I have no idea how it actually works. On the other hand I use git for almost all of my projects. Git is much more familiar for me than SVN and I’m certain there are others out there in the same boat.
Using git instead removes a barrier of entry to core contribution that previously blocked me from actively being involved during development.