Add revert patch support to the rbt patch command

Review Request #7084 — Created March 19, 2015 and submitted — Latest diff uploaded

Information

RBTools
release-0.7.x
cb727d6...

Reviewers

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.