Fix merge-to-branch python port.

This fixes a forgotten parameter when calling the script's toplevel method and adds the same usage message as in the bash script.

BUG=
TBR=ulan@chromium.org

Review URL: https://codereview.chromium.org/164373015

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19450 ce2b1a6d-e550-0410-aec6-3dcde31c8c00