For a variety of reasons, we decided to move from an Assembla-hosted Git repository to a Github one, for our core development. The addons repository and others are not touched by this.
Some of the reasons for the switch:
This means that the new Git repository is now available for cloning in Git at https://github.com/ImpressCMS/impresscms.git
To browse the repository, use https://github.com/ImpressCMS/impresscms
The addons repository needs some more work, but @fiammybe and @mekdrop are working on that as well. Expect some news about that move in the near future.