The existing code would set a namespace of "" (empty string) with
GIT_NAMESPACE unset.  In a repository where refs/heads/namespaces/
exists, that can produce incorrect results.  Detect that case and avoid
setting the namespace at all.

Since that makes the last assignment to error conditional, and the
previous assignment can potentially get GIT_ENOTFOUND, set error to 0
explicitly to prevent the call from incorrectly failing with
GIT_ENOTFOUND.