Fix test failure in test_main.TestCookiecutterRepoArg.test_cookiecutter_git.

For some unknown reason, test_main.TestCookiecutterRepoArg.test_cookiecutter_git
started failing on my system for Python 2.6 & 3.3 (both in tox and in isolated
2.6 & 3.3 virtualenvs). I believe this test failure was completely unrelated to
@benjixx's file permissions changes.

This commit is a bit hackish, so I added detailed explanations inline. If anyone
happens to have any insight about why this might have started happening or how
to improve this, I am interested.