How to design a SQL db with undo-redo?

Question

I'm trying to figure out how to design my DB tables to allow Undo-Redo.

Pretend you have a tasks table with the following structure:

id <int>
title <varchar>
memo <string>
date_added <datetime>
date_due <datetime>

Now assume that over a few days and multiple log-ins that several edits have taken place; but a user wants to go back to one of the versions.

1. Would you have a separate table tracking the changes - or - would you try to keep the changes within the tasks table ("ghost" rows, for lack of a better term)?
2. Would you track all of the columns or just the ones that changed each time?

If it matters, I'm using MySQL. Also, if it matters, I'd like to be able to show the history (ala Photoshop) and allow a user to switch to any version.

Bonus question: Would you save the whole memo cell on a change or would you try to save the delta only? Reason I ask is because the memo cell could be large and only a single word or character might be changed each revision. Granted, saving the delta would require parsing, but if undos aren't expected very often, wouldn't it be better to save space rather than processing time?

Thank you for your help.