Make v8 releases script ready for chromium git switch.

Make v8 releases script ready for chromium git switch.

This uses chromium commit numbers that are supported after
the chromium git switch. Most functionality is copied from
the bot_update.py script from tools/build.

This also enables mapping bleeding edge revisions to chromium
revisions (previously the data was only retrieved for trunk).

TEST=script_test.py
TEST=tools/push-to-trunk/releases.py -c /path/to/chromium/src --branch recent

TBR=jarin@chromium.org

Review URL: https://codereview.chromium.org/497043002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23319 ce2b1a6d-e550-0410-aec6-3dcde31c8c00