"git-checkout -b frotz/nitfol master" failed to create
$GIT_DIR/refs/heads/frotz/nitfol but went ahead and updated
$GIT_DIR/HEAD to point at it, resulting in a corrupt repository.
Exit when we cannot create the new branch with an error status.
While we are at it, there is no reason to forbid subdirectories
in refs/heads, so make sure we handle that correctly.