rebase: fix regression in rebase.useBuiltin=false test mode

Fix a recently introduced regression in c762aada ("rebase -x: sanity
check command", 2019-01-29) triggered when running the tests with
GIT_TEST_REBASE_USE_BUILTIN=false. See 62c23938 ("tests: add a
special setup where rebase.useBuiltin is off", 2018-11-14) for how
that test mode works.

As discussed on-list[1] it's not worth it to implement the sanity
check in the legacy rebase code, we plan to remove it after the 2.21
release. So let's do the bare minimum to make the tests pass under the
GIT_TEST_REBASE_USE_BUILTIN=false special setup.

1. https://public-inbox.org/git/xmqqva1nbeno.fsf@gitster-ct.c.googlers.com/Signed-off-by: NÆvar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

t/t3404-rebase-interactive.sh

@@ -149,10 +149,12 @@ test_expect_success 'rebase -i with the exec command checks tree cleanness' '
 
 test_expect_success 'rebase -x with empty command fails' '
 	test_when_finished "git rebase --abort ||:" &&
-	test_must_fail git rebase -x "" @ 2>actual &&
+	test_must_fail env GIT_TEST_REBASE_USE_BUILTIN=true \
+		git rebase -x "" @ 2>actual &&
 	test_write_lines "error: empty exec command" >expected &&
 	test_i18ncmp expected actual &&
-	test_must_fail git rebase -x " " @ 2>actual &&
+	test_must_fail env GIT_TEST_REBASE_USE_BUILTIN=true \
+		git rebase -x " " @ 2>actual &&
 	test_i18ncmp expected actual
 '
 
@@ -160,7 +162,8 @@ LF='
 '
 test_expect_success 'rebase -x with newline in command fails' '
 	test_when_finished "git rebase --abort ||:" &&
-	test_must_fail git rebase -x "a${LF}b" @ 2>actual &&
+	test_must_fail env GIT_TEST_REBASE_USE_BUILTIN=true \
+		git rebase -x "a${LF}b" @ 2>actual &&
 	test_write_lines "error: exec commands cannot contain newlines" \
 			 >expected &&
 	test_i18ncmp expected actual