git-codereview: allow multiple commit hashes for submit

This adds support to the submit subcommand for passing and submitting
multiple commit hashes.

Change-Id: Id030b07cac21d11acbeb8015fde9cb9e1acb1479
Reviewed-on: https://go-review.googlesource.com/16674
Reviewed-by: Andrew Gerrand <adg@golang.org>
4 files changed