Base updates often fail because of existing files.

The current approach for updating the cookiecutter-base branch is quite dangerous. For example, if one changes the name of the project, then running cookiecutter will put the updates in a sub-folder (because the symlink will be invalid now). Even if the update was correctly done in place, the current approach of would keep both src/old_project and src/new_project.

I think a much better approach would be to delete everything except .cookiecutter.yaml and recreate the environment from scratch before running hg addremove. This needs testing.