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 add00ba2de (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>

t/helper/test-date.c

@@ -35,7 +35,7 @@ static void show_human_dates(const char **argv)
 
 static void show_dates(const char **argv, const char *format)
 {
-	struct date_mode mode;
+	struct date_mode mode = DATE_MODE_INIT;
 
 	parse_date_format(format, &mode);
 	for (; *argv; argv++) {