Return 0 (instead of -1) for zero-length components. Move the
interpretation of zero-length components as illegal to
check_refname_format().
This will make it easier to extend check_refname_format() to also
check whether directory names are valid.
Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>