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>

t/t4200-rerere.sh

@@ -113,8 +113,11 @@ mkdir $rr2
 echo Hello > $rr2/preimage
 
 case "$(date -d @11111111 +%s 2>/dev/null)" in
-[1-9]*)
-	# it is a recent GNU date. good.
+11111111)
+	# 'date' must be able to take arbitrary input with @11111111 notation.
+	# for this test to succeed.  We should fix this part using more
+	# portable script someday.
+
 	now=$(date +%s)
 	almost_15_days_ago=$(($now+60-15*86400))
 	just_over_15_days_ago=$(($now-1-15*86400))
@@ -124,31 +127,25 @@ case "$(date -d @11111111 +%s 2>/dev/null)" in
 	predate2="$(date -d "@$almost_15_days_ago" +%Y%m%d%H%M.%S)"
 	postdate1="$(date -d "@$just_over_60_days_ago" +%Y%m%d%H%M.%S)"
 	postdate2="$(date -d "@$just_over_15_days_ago" +%Y%m%d%H%M.%S)"
+
+	touch -m -t "$predate1" $rr/preimage
+	touch -m -t "$predate2" $rr2/preimage
+
+	test_expect_success 'garbage collection (part1)' 'git rerere gc'
+
+	test_expect_success 'young records still live' \
+		"test -f $rr/preimage -a -f $rr2/preimage"
+
+	touch -m -t "$postdate1" $rr/preimage
+	touch -m -t "$postdate2" $rr2/preimage
+
+	test_expect_success 'garbage collection (part2)' 'git rerere gc'
+
+	test_expect_success 'old records rest in peace' \
+		"test ! -f $rr/preimage -a ! -f $rr2/preimage"
 	;;
-*)
-	# it is not GNU date. oh, well.
-	predate1="$(date +%Y%m%d%H%M.%S)"
-	predate2="$(date +%Y%m%d%H%M.%S)"
-	postdate1='200610010000.00'
-	postdate2='200612010000.00'
 esac
 
-touch -m -t "$predate1" $rr/preimage
-touch -m -t "$predate2" $rr2/preimage
-
-test_expect_success 'garbage collection (part1)' 'git rerere gc'
-
-test_expect_success 'young records still live' \
-	"test -f $rr/preimage -a -f $rr2/preimage"
-
-touch -m -t "$postdate1" $rr/preimage
-touch -m -t "$postdate2" $rr2/preimage
-
-test_expect_success 'garbage collection (part2)' 'git rerere gc'
-
-test_expect_success 'old records rest in peace' \
-	"test ! -f $rr/preimage -a ! -f $rr2/preimage"
-
 test_done