Empty ranges -L,+0 and -L,-0 are nonsensical in the context of blame yet
they are accepted (in fact, both are interpreted as -L1,Y where Y is
end-of-file). Report them as invalid.
Signed-off-by: Eric Sunshine <sunshine@sunshineco.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>