This is a hack to support the most common git workflow: keeping
a local branch named 'master' which is a replica of 'origin/master'
with no local changes, and basing all feature branches off of
that.

Test:
* created master branch, created feature branch, uploaded CL correctly
* created master branch, landed change on master branch, created
  feature branch, uploaded CL, CL contains change on master too
* created non-master branch, created feature branch, failed to
  upload CL

Bug: 682104
Change-Id: I8481b787e6dcab162d2846c07f1ddad950f491e0
Reviewed-on: https://chromium-review.googlesource.com/464107Reviewed-by: Andrew Bonventre <andybons@google.com>
Commit-Queue: Aaron Gable <agable@chromium.org>