strbuf: change an always NULL/"" strbuf_addftime() param to bool

strbuf: change an always NULL/"" strbuf_addftime() param to bool

strbuf_addftime() allows callers to pass a time zone name for
expanding %Z. The only current caller either passes the empty string
or NULL, in which case %Z is handed over verbatim to strftime(3).
Replace that string parameter with a flag controlling whether to
remove %Z from the format specification. This simplifies the code.

Commit-message-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>