Show a smarter diff

[benjaminjkraft]

This mostly only applies to usage as a library, although in principle one could hit it with a large multiline regexp. It would be really nice if instead of just showing the entire removed block as red and the entire added block as green, codemod could show a more concise diff. This would be especially useful for codemods that reorder lines, for example by moving a line up a few lines -- right now they show a big unreadable block of red and green, but they could just show a single red and a single green line.

(In an ideal world, they could even allow diff-splitting, like git add -p, but I don't think that's particularly compatible with the current Patch architecture.)