Add "Z" as an alias for the timezone "UTC"

Add "Z" as an alias for the timezone "UTC"

The name "Z" for the UTC timezone is required to properly parse ISO 8601
timestamps.  Add it to the list of recognized timezones.

Because timezone names can be shorter than 3 letters, loosen the
restriction in match_alpha() that used to require at least 3 letters to
match to allow a short timezone name as long as it matches exactly.  Prior
to the introduction of the "Z" zone, this already affected the timezone
"NT" (Nome).

Signed-off-by: Marcus Comstedt <marcus@mc.pp.se>
Reviewed-by: Jay Soffian <jaysoffian@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>