t2019-checkout-ambiguous-ref.sh: depend on C_LOCALE_OUTPUT

The t2019-checkout-ambiguous-ref.sh tests added in v1.7.4.3~12^2
examines the output for a translatable string, and must be marked
with C_LOCALE_OUTPUT; otherwise, GETTEXT_POISON=YesPlease tests
will break.
Signed-off-by: NÆvar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

t/t2019-checkout-ambiguous-ref.sh

@@ -29,7 +29,7 @@ test_expect_success 'checkout chooses branch over tag' '
 	test_cmp expect file
 '
 
-test_expect_success 'checkout reports switch to branch' '
+test_expect_success C_LOCALE_OUTPUT 'checkout reports switch to branch' '
 	grep "Switched to branch" stderr &&
 	! grep "^HEAD is now at" stderr
 '
@@ -51,7 +51,7 @@ test_expect_success VAGUENESS_SUCCESS 'checkout chooses branch over tag' '
 	test_cmp expect file
 '
 
-test_expect_success VAGUENESS_SUCCESS 'checkout reports switch to branch' '
+test_expect_success VAGUENESS_SUCCESS,C_LOCALE_OUTPUT 'checkout reports switch to branch' '
 	grep "Switched to branch" stderr &&
 	! grep "^HEAD is now at" stderr
 '