Fix patch creation in merge-to-branch script.

Patch white space got stripped, which lead to failures when applying the patch.

- Refine test to reveal the problem
- Create a separate method for retrieving the patch that doesn't strip white space

R=jkummerow@chromium.org

Review URL: https://codereview.chromium.org/181603002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19566 ce2b1a6d-e550-0410-aec6-3dcde31c8c00