There is a good question on Stack Overflow about it: http://stackoverflow.com/questions/132478/how-to-perform-string-diffs-in-java
Here is a good tutorial on how to send email with CQ : http://forums.adobe.com/message/5309387
thanks for your answer!
I ask myself if it makes sense to use other java diff library where the Diff function is already implemented in CQ. I was reading about Diffinfo and Diffservice but could not find any example or tutorial how to modify/implement this functionality.
Some silly code that shows this service usage:
DiffService diffService = sling.getService(DiffService.class);
String diff = diffService.diff(currentPage.getPageManager().getRevisions().., currentPage.getPageManager().getRevisions()...);