Make trackGit idempotent

[Chris Warburton]

Check if the current repo already has remotes, etc. Take the name from
there if found, otherwise ask (like we do now).

Only perform modifications (adding remotes, etc.) if they've not already
been done.

Thankfully some of the hard work is already done in the local and remote
'check' scripts, and those are idempotent (but slow; meh).