If you take the 18th, apply the 19th changes, then technically you can then compare 19th to 20th
Since you do have a 18th backup, you could do it a lot either than me, since I'd have to manually do it reading through the changes you posted. (Easier to do in the program since you can Search For Next Difference, hehe. Plus, you can't copy/paste one side from the HTML differences page)
*edit* Unless of course you could post a 18th zip, and I'd be happy to do it for you =)