Make merge_to_branch more git-friendly.

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

Review URL: https://codereview.chromium.org/702843002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@25154 ce2b1a6d-e550-0410-aec6-3dcde31c8c00