Well, whatever you do you need to be able to pass your office proxy, so it completely depends on what kind of traffic it allows. You will want to take this up with your system administrator. Surely they can make an exception for something like GitHub. If they don't allow you to work on hobby projects on office material then the point is moot anyway, and we can't help you with that.
You could try e-mailing yourself a patch that you got from git format-patch, and then apply it, which is easier than overwriting the files manually.