Fix change log creation in push script.

BUG=chromium:408523
LOG=n
TBR=tandrii@chromium.org
TEST=script_test.py

Review URL: https://codereview.chromium.org/710593003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@25244 ce2b1a6d-e550-0410-aec6-3dcde31c8c00