Get source -link should point always point to the HEAD

Get source -link should always point to the HEAD. not the current HEAD. This way users could share a link which will always contain the newest version of the software in tarball.

Take example from Github.

This would be especially great with Python projects. You could then always install the newest version with easy_install.

eg.

{{

{\\ $ easy_install http://bitbucket.org/user/app/get/head.tar.bz2\\ }}

}