git-review: reject empty messages in commit-msg hook

Fixes #9336

Change-Id: Ie145c116ed91cfac0eb2061c448b08c5b9595d19
Reviewed-on: https://go-review.googlesource.com/1612
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
2 files changed
tree: c173a5c6a09d13baeea3f8417c2fff83b36147f9
  1. README
  2. git-review/