Cloning this repository:

git clone git://gitorious.org/w3c-bibtex/mainline.git mainline
cd mainline

Add this repository as a remote to an existing local repository:

git remote add mainline git://gitorious.org/w3c-bibtex/mainline.git
git fetch mainline
git checkout -b my-local-tracking-branch mainline/master_or_other_branch