t3419: stop losing return code of git command

Fix invocation of git command so its exit codes is not lost within
a non-assignment command substitution.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t3419-rebase-patch-id.sh

@@ -80,7 +80,8 @@ do_tests () {
 		git commit -q -m "change big file again" &&
 		git checkout -q other^{} &&
 		git rebase master &&
-		test_must_fail test -n "$(git rev-list master...HEAD~)"
+		git rev-list master...HEAD~ >revs &&
+		test_must_be_empty revs
 	'
 
 	test_expect_success $pr 'do not drop patch' '