I found a patch I want here:
I can see the diff, but what do I do now? How can I get that code into my Wordpress? Do I have to copy it manually? Is there anywhere to download it from?
You can download the diff file from the bottom of the diff page.
The program to apply a diff is called "patch". It's widely available for various platforms.
Example usages: http://en.wikipedia.org/wiki/Patch_(Unix)#Usage_examples