Rietveld rejects tryjobs from non-latest patchsets anyway,
change logic to always use latest patchset.

R=iannucci@chromium.org

Review URL: https://chromiumcodereview.appspot.com/22415012

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@216023 0039d316-1c4b-4281-b951-d872f2087c98