t4200: skip gc-rerere test on systems with non GNU date.

Quite nonstandard "date -d @11111111 +%s" does not even fail on
OpenBSD but gives the current date in "seconds since epoch"
format, which is useless for the purpose of this test.  We want
to make sure that this returns exactly the same input before
proceeding.

Signed-off-by: Junio C Hamano <junkio@cox.net>