git-codereview: reject empty commits in mail

Fixes golang/go#10753.

Change-Id: I224a411177b3d198576a06bc69da9894b7b67c34
Reviewed-on: https://go-review.googlesource.com/19799
Run-TryBot: Andrew Gerrand <adg@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>
4 files changed