Add merge-to-branch python port.

- To ease a line-by-line review, the script is intentionally close to the former bash version
- Disambiguate the existing "-r" option for reviewer in the other scripts
- The options design will be refactored in a follow up CL

TEST=python -m unittest test_scripts.ScriptTest.testMergeToBranch
R=ulan@chromium.org

Review URL: https://codereview.chromium.org/163183004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19443 ce2b1a6d-e550-0410-aec6-3dcde31c8c00