On git repository there are two protocols to clone Tuleap sources: ssh & http
https://tuleap.net/plugins/git/tuleap/tuleap/stable
However, when you use the given url for the http protocol, an error is displayed:
$> git clone http://tuleap.net/plugins/git/tuleap/tuleap/stable.git
Cloning into 'stable'...
fatal: repository 'http://tuleap.net/plugins/git/tuleap/tuleap/stable.git/' not found
This works fine if you change http to https.