Return the root of the directory corresponding to the desired environment,
    or None if the environment was not found.

def _env_root(repo, saltenv):
    '''
    Return the root of the directory corresponding to the desired environment,
    or None if the environment was not found.
    '''
    # If 'base' is desired, look for the trunk
    if saltenv == 'base':
        trunk = os.path.join(repo['repo'], repo['trunk'])
        if os.path.isdir(trunk):
            return trunk
        else:
            return None

    # Check branches
    branches = os.path.join(repo['repo'], repo['branches'])
    if os.path.isdir(branches) and saltenv in os.listdir(branches):
        return os.path.join(branches, saltenv)

    # Check tags
    tags = os.path.join(repo['repo'], repo['tags'])
    if os.path.isdir(tags) and saltenv in os.listdir(tags):
        return os.path.join(tags, saltenv)

    return None