t6013: replace use of 'tac' with equivalent Perl

'tac' is not available everywhere, so substitute the equivalent Perl
code 'print reverse <>'.  Noticed by Brian Gernhardt.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t6013-rev-list-reverse-parents.sh

@@ -25,7 +25,7 @@ test_expect_success 'set up --reverse example' '
 
 test_expect_success '--reverse --parents --full-history combines correctly' '
 	git rev-list --parents --full-history master -- foo |
-		tac > expected &&
+		perl -e "print reverse <>" > expected &&
 	git rev-list --reverse --parents --full-history master -- foo \
 		> actual &&
 	test_cmp actual expected
@@ -33,7 +33,7 @@ test_expect_success '--reverse --parents --full-history combines correctly' '
 
 test_expect_success '--boundary does too' '
 	git rev-list --boundary --parents --full-history master ^root -- foo |
-		tac > expected &&
+		perl -e "print reverse <>" > expected &&
 	git rev-list --boundary --reverse --parents --full-history \
 		master ^root -- foo > actual &&
 	test_cmp actual expected