Briefcase doesn't ensure cookiecutter template is correct branch before use

[freakboy3742]

(Discovered with @rogueleaderr during pair programming session)

If you generate a Briefcase app with a particular version of Python (e.g., run python setup.py macos under Python 3.6, and then run a second time under Python 3.5), the cookiecutter app template isn't switched to the correct branch.

This is because the cloning process only runs if the directory does not exist. On subsequent runs, the cookiecutter repo is pulled, but no branch switch takes place.

Cookiecutter does have logic in place to re-clone and switch branches, but this only gets called if a complete re-clone is executed. This may not be a suitable solution for Briefcase, because it requires the user be online when running Briefcase.

[freakboy3742]

The 0.3 development branch addresses this by tweaking the wrapper around cookiecutter's template usage behavior.