Fixing remote branch creation (only want master in github).

If you create a branch 'a', then branch off that to 'b', then running 'git cl push' from 'b' will create a branch 'b' in origin instead of just pushing to master.

This change to the PRESUBMIT would set the upstream to master before pushing.

R=jackhou@chromium.org, tapted@chromium.org

Review URL: https://codereview.appspot.com/49670045