Refactor ChangeLog generation for push-to-trunk script.

This extracts the Git-independent part of the change log body generation. This CL intends no change in behavior.

R=jkummerow@chromium.org

Review URL: https://codereview.chromium.org/61263011

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@17829 ce2b1a6d-e550-0410-aec6-3dcde31c8c00