parse-options.c: introduce OPT_DATE

There are quite a few places that will need to call approxidate(),
when they'll adopt the parse-options system, so this patch adds the
function parse_opt_approxidate_cb(), used by OPT_DATE.

Signed-off-by: Michele Ballabio <barra_cuda@katamail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

parse-options.c

@@ -409,3 +409,10 @@ int parse_opt_abbrev_cb(const struct option *opt, const char *arg, int unset)
 	*(int *)(opt->value) = v;
 	return 0;
 }
+
+int parse_opt_approxidate_cb(const struct option *opt, const char *arg,
+			     int unset)
+{
+	*(unsigned long *)(opt->value) = approxidate(arg);
+	return 0;
+}