date.c: improve guess between timezone offset and year.

When match_digit() guesses a four-digit string to tell if it is
a year or a timezone, it did not consider that some real-world
places have UTC offsets equal to +1400.

   $ date; TZ=UTC0 date; TZ=Pacific/Kiritimati date
   Wed Jun  7 23:25:42 PDT 2006
   Thu Jun  8 06:25:42 UTC 2006
   Thu Jun  8 20:25:42 LINT 2006

Signed-off-by: Paul Eggert <eggert@CS.UCLA.EDU>
Signed-off-by: Junio C Hamano <junkio@cox.net>