t6041: do not compress backup tar file

The test uses the 'z' option, i.e. "compress the output while at
it", which is GNUism and not portable.

Reported-by: Armin Kunaschik <megabreit@googlemail.com>
Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: stefanbeller

t/t6041-bisect-submodule.sh

@@ -8,7 +8,7 @@ test_description='bisect can handle submodules'
 git_bisect () {
 	git status -su >expect &&
 	ls -1pR * >>expect &&
-	tar czf "$TRASH_DIRECTORY/tmp.tgz" * &&
+	tar cf "$TRASH_DIRECTORY/tmp.tar" * &&
 	GOOD=$(git rev-parse --verify HEAD) &&
 	git checkout "$1" &&
 	echo "foo" >bar &&
@@ -20,7 +20,7 @@ git_bisect () {
 	git bisect start &&
 	git bisect good $GOOD &&
 	rm -rf * &&
-	tar xzf "$TRASH_DIRECTORY/tmp.tgz" &&
+	tar xf "$TRASH_DIRECTORY/tmp.tar" &&
 	git status -su >actual &&
 	ls -1pR * >>actual &&
 	test_cmp expect actual &&