The code is hosted in the freedesktop.org GitLab instance.
You can clone the repository using this command:
git clone https://gitlab.freedesktop.org/zeitgeist/zeitgeist.git
To submit a change, use a fork of the Git repository and submit a pull request with the GitLab interface.
You can link the issue your pull request is fixing by adding
fixes #XXX (XXX being the number of the issue) to the description of your pull request.