date.c: fix unsigned time_t comparison

tm_to_time_t() returns (time_t)-1 when it sees an error.  On
platforms with unsigned time_t, this value will be larger than any
valid timestamp and will break the "Is this older than 10 days in
the future?" check.

Signed-off-by: Mike Gorchak <mike.gorchak.qnx@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

date.c

@@ -383,7 +383,7 @@ static int is_date(int year, int month, int day, struct tm *now_tm, time_t now,
 		 * sense to specify timestamp way into the future.  Make
 		 * sure it is not later than ten days from now...
 		 */
-		if (now + 10*24*3600 < specified)
+		if ((specified != -1) && (now + 10*24*3600 < specified))
 			return 0;
 		tm->tm_mon = r->tm_mon;
 		tm->tm_mday = r->tm_mday;