p3400: stop using tac(1)

b3dfeebb92 (rebase: avoid computing unnecessary patch IDs, 2016-07-29)
added a perf test that calls tac(1) from GNU core utilities.  Support
systems without it by reversing the generated list using sort -nr
instead.  sort(1) with options -n and -r is already used in other tests.

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/perf/p3400-rebase.sh

@@ -18,7 +18,7 @@ test_expect_success 'setup rebasing on top of a lot of changes' '
 		test_tick &&
 		git commit -m commit$i unrelated-file$i &&
 		echo change$i >unrelated-file$i &&
-		test_seq 1000 | tac >>unrelated-file$i &&
+		test_seq 1000 | sort -nr >>unrelated-file$i &&
 		git add unrelated-file$i &&
 		test_tick &&
 		git commit -m commit$i-reverse unrelated-file$i ||