git-codereview: diagnose local tracking branch ahead of remote

Otherwise the git change error is a bit confusing.

Change-Id: I8bacac175417231284a238a078e5ea9513866943
Reviewed-on: https://go-review.googlesource.com/2786
Reviewed-by: Andrew Gerrand <adg@golang.org>
2 files changed
tree: 098ca554b4f3ad9ab01931e3b1d3392ecda6a7c5
  1. .gitattributes
  2. README
  3. git-codereview/