Patch File doesn't include Unstaged Changes

Not an issue with patch filter itself, but we need documentation on the best way to set up this repo for our users. There is nothing in our readme for this.

Identified when testing https://github.com/checkstyle/patch-filters/issues/352 , I made changes to the repo but did not commit them. They did not appear in `show.patch` which wasn't expected.

I test my changes before committing them all the time to ensure I am not committing something bad and losing my original work. I think we should find some way to include unstaged changes as well with the top commit.

Repo's patch file creation: https://github.com/checkstyle/eclipse-cs/blob/3ab5c034979069359fa5505dd34117af374a0328/pom.xml#L139-L157