git-blame -L is 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 rejected but is
nevertheless accepted. Demonstrate this shortcoming.
Signed-off-by: Eric Sunshine <sunshine@sunshineco.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>