Improve branch creation in release scripts.

Use 'git new-branch' for creating new branches.

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

Review URL: https://codereview.chromium.org/680173003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24940 ce2b1a6d-e550-0410-aec6-3dcde31c8c00