Running rbt patch with a Mercurial repository failed with the error

"patch: unrecognized option '--config'" because _execute() in

rbtools/clients/mercurial.py appends a --config option for loading hgrc files.

This option is used for hg commands, but is not a valid option for patch. To

fix this, we run patch by calling execute().

Ran unit tests.
Ran rbt patch with a Mercurial repository, and saw that the patch was

  applied successfully.