Ability to diff the source code of forks of a scratch

[EllipticEllipsis]

Essentially so that you can find out what the kind person who matched your function actually changed. Will have to include the context as well since that sometimes needs to be edited. I'd imagine that in most cases a git diff-style display would be sufficient, although of course there will be ones where a complete rewrite was required, although that'll probably be obvious. Probably should also include ability to ignore whitespace, since people will fiddle with it to get it to their liking when working on scratches, although not essential.

[bates64]

Related to #361