Make trackGit add github remote to working copies

[Chris Warburton]

If we have multiple working copies of a project on different machines,
we want some way to merge changes across all of them.

nixos.local is the best place to push things out to all mirrors, etc.
since it's got SSH access to chriswarbo.net and GitHub.

It's useful for other machines to push changes to GitHub, since it's on
the public Internet (unlike nixos.local), is easy enough to add and
remove SSH to/from, and doesn't have shell access (unlike if we added
SSH keys to chriswarbo.net).

For this reason, working copies on nixos.local should be able to pull
changes (made on other machines) from GitHub; then push them everywhere
like we do at the moment.

To do this we need to add the github repo as remote, checking if it's
there first.