Teach push script to commit with roll account.

BUG=410721
LOG=n
TEST=script_test.py
TBR=tandrii@chromium.org

Review URL: https://codereview.chromium.org/693333002

Cr-Commit-Position: refs/heads/master@{#25046}
git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@25046 ce2b1a6d-e550-0410-aec6-3dcde31c8c00