Reland "Maintain change log file directly on trunk branch in push-to-trunk."

This uses the following approach to modify the trunk change log:
- Calculate the new change log entries
- Apply changes to the bleeding edge change log (this will be removed in a follow up CL)
- Apply the diff between BE and trunk to trunk (this includes the diff of the change log)
- Reset the change log to the version on trunk
- Reapply the new change log entries

R=jarin@chromium.org

Review URL: https://codereview.chromium.org/196473012

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19953 ce2b1a6d-e550-0410-aec6-3dcde31c8c00