Use GitHub endpoint for Gitosis

Fixes:

    git clone git://eagain.net/gitosis.git
    Cloning into 'gitosis'...
    fatal: unable to connect to eagain.net:
    eagain.net[0: 192.237.217.227]: errno=Connection refused
    eagain.net[1: 2001:4800:7813:516:fa64:3a5d:ff04:7851]: errno=Network is
      unreachable