Fix pre-review hook once and for all

Don't rely on a regex to get the project name from git. Just specify
project name in the script instead. Replace a cryptic "O=1" with
"o=LABELS". [1]

Also, if Gerrit API returns no changes (which is very unlikely), show
a warning message.

[1] http://gerrit-review.googlesource.com/Documentation/rest-api-changes.html#list-changes
Change-Id: Iec52501dfc6ffc84546fbf0d2fd41131a706e07d
Merge "Fix pre-review hook once and for all"