strbuf: let strbuf_addftime handle %z and %Z itself

strbuf: let strbuf_addftime handle %z and %Z itself

There is no portable way to pass timezone information to strftime.  Add
parameters for timezone offset and name to strbuf_addftime and let it
handle the timezone-related format specifiers %z and %Z internally.

Callers can opt out for %Z by passing NULL as timezone name.  %z is
always handled internally -- this helps on Windows, where strftime would
expand it to a timezone name (same as %Z), in violation of POSIX.
Modifiers are not handled, e.g. %Ez is still passed to strftime.

Use an empty string as timezone name in show_date (the only current
caller) for now because we only have the timezone offset in non-local
mode.  POSIX allows %Z to resolve to an empty string in case of missing
information.

Helped-by: Ulrich Mueller <ulm@gentoo.org>
Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>