Don't use a temp file for changelog in push-to-trunk.

This is part of getting rid of the ChangeLog on bleeding_edge and directly modifying it on trunk.

BUG=
R=ulan@chromium.org

Review URL: https://codereview.chromium.org/195183003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19832 ce2b1a6d-e550-0410-aec6-3dcde31c8c00