Add review script

I've been using this script for a while to update commits before
pushing them to the main repository. It copies all commits in a
range to another branch using git cherry-pick and starts an editor
to modify the Reviewed-by: line(s) for each commit. The script is
certainly not perfect, but it does the job.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>