If someone will use the system-wide rbt there is no need
to install a user installed rbt again.

Also download of the patch is necessary for binary diffs
or if the repository is in a subdirectory. So applying with "rbt patch"
would fail. This allows so get more control over the patching.

Added plugin to local jenkins. Tried with and without downloadOnly and with and without download Rbt without a problem.
Also added unit tests for it.