date API: provide and use a DATE_MODE_INIT

date API: provide and use a DATE_MODE_INIT

Provide and use a DATE_MODE_INIT macro. Most of the users of struct
date_mode" use it via pretty.h's "struct pretty_print_context" which
doesn't have an initialization macro, so we're still bound to being
initialized to "{ 0 }" by default.

But we can change the couple of callers that directly declared a
variable on the stack to instead use the initializer, and thus do away
with the "mode.local = 0" added in add00ba2de9 (date: make "local"
orthogonal to date format, 2015-09-03).

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>