Teach chromium_roll script to roll v8 git hashes.

TEST=script_test.py
R=hinoka@google.com

Review URL: https://codereview.chromium.org/504793002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23379 ce2b1a6d-e550-0410-aec6-3dcde31c8c00