Change the location zit is cloned from.

This is a bit of shameful self-promotion of my own semi-fork of zit: I
want to make it easier on users with a man page, and hopefully easier
on packagers (like myself), or users on other systems, to install it
with a Makefile.

I had tried to push both of these upstream, but the zit author
maintains a philosophy that zit itself is maintained in a manner
compatible with zit: a single-file repository. I respect this choice,
but I believe that violating that philosophy for the benefit of users
is more valuable.