In addition to approxidate-style values ("2.months.ago", "yesterday"),
consumers of 'gc.*expire*' configuration variables also accept and
respect 'now' ("do it immediately") and 'never' ("suppress entirely").
Suggested-by: Michael Haggerty <mhagger@alum.mit.edu> Signed-off-by: Eric Sunshine <sunshine@sunshineco.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>