t0006: skip "far in the future" test when unsigned long is not long enough

Git's source code refers to timestamps as unsigned longs.  On 32-bit
platforms, as well as on Windows, unsigned long is not large enough
to capture dates that are "absurdly far in the future".

While we can fix this issue properly by replacing unsigned long with
a larger type, we want to be a bit more conservative and just skip
those tests on the maint track.

Signed-off-by: Jeff King <peff@peff.net>
Helped-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/t/test-lib.sh b/t/test-lib.sh
index 51e4a88..4595734 100644
--- a/t/test-lib.sh
+++ b/t/test-lib.sh
@@ -1098,3 +1098,12 @@
 }
 
 test_lazy_prereq CMDLINE_LIMIT 'run_with_limited_cmdline true'
+
+build_option () {
+	git version --build-options |
+	sed -ne "s/^$1: //p"
+}
+
+test_lazy_prereq LONG_IS_64BIT '
+	test 8 -le "$(build_option sizeof-long)"
+'