Since inception, git-blame -L has been documented as accepting 1-based
line numbers. When handed a line number less than 1, -L's behavior is
undocumented and undefined; it's also nonsensical and should be
diagnosed as an error. Do so.
Signed-off-by: Eric Sunshine <sunshine@sunshineco.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>