Add check to PRESUBMIT.py that local branch matches uploaded patch set

"git cl push" will push whatever is in the local branch instead
of using the patch in the Rietveld issue.
This presubmit check is to work around this behaviour.

This patch is based on https://codereview.appspot.com/40650046/ .

R=jackhou@chromium.org

Review URL: https://codereview.appspot.com/41670043