Pair up for collaboration and conflicts lessons

I've done this in a previous workshop, and I think it will be clearer for the students than 'collaborating' between two repositories on a single computer (I haven't actually tried teaching that, but it seems counterintuitive).

The downside is that several diagrams are now inapplicable. I've drawn a couple of new diagrams to go in their place, but I haven't got time to replace them all, so I've just removed some.