Add corresponding support to scm.SVN.Revert() to clobber the svn:ignore'd files.

This makes the commit queue remove all the unversioned files, which could have
improved its stability. It failed in practice to improve it but it's still a
good thing to do overall.

R=petermayo@chromium.org
BUG=
TEST=


Review URL: http://codereview.chromium.org/10355014

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@135333 0039d316-1c4b-4281-b951-d872f2087c98