Make releases script ready for the new git workflow.

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

Review URL: https://codereview.chromium.org/646383002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24558 ce2b1a6d-e550-0410-aec6-3dcde31c8c00