Make releases script more git friendly.

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

Review URL: https://codereview.chromium.org/682863006

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@25022 ce2b1a6d-e550-0410-aec6-3dcde31c8c00