This package uses the upstream git repository as its source because the released
version is really too old.
Original source tarball is created by cloning the upstream git repository
(branch master), removing all git-specific files, and removing directories with
problematic licenses issues.
All changes to the upstream code are maintained as quilt patches and can be
found in debian/patches.
To get the an equivalent tarball as the one uploaded to the archive, you can use
the get-orig-source target of debian/rules (per Debian Policy §4.9):

    debian/rules get-orig-source
