commit-graph tests: fix error-hiding graph_git_two_modes() helper

The graph_git_two_modes() helper added in 177722b (commit:
integrate commit graph with commit parsing, 2018-04-10) didn't
&&-chain its "git commit-graph" invocations, which as can be seen with
SANITIZE=leak will happily mark tests as passing if both of these
commands die, since test_cmp() will be comparing two empty files.

Signed-off-by: Ævar Arnfjörð Bjarmason <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: avar

t/t5318-commit-graph.sh

@@ -51,8 +51,8 @@ test_expect_success 'create commits and repack' '
 '
 
 graph_git_two_modes() {
-	git -c core.commitGraph=true $1 >output
-	git -c core.commitGraph=false $1 >expect
+	git -c core.commitGraph=true $1 >output &&
+	git -c core.commitGraph=false $1 >expect &&
 	test_cmp expect output
 }