The rbt patch command is now capable of reverting patches via the

-R/--revert command line flag.

Ran unit tests.
Tested rbt patch -R with CVS, Git, Perforce, and SVN.
Applied a patch with Perforce that had empty files and was able to

revert it with rbt patch -R.
Applied a patch with SVN that had empty files and was able to revert

it with rbt patch -R.
Running rbt patch -R when using a Mercurial repository results in an

error as expected.
I am not able to verify that this works with Plastic or ClearCase, but

it should work because they both use the default apply_patch method.