t1091: disable split index

In 61feddcdf2 (tests: disable GIT_TEST_SPLIT_INDEX for sparse index
tests, 2021-08-26), it was already called out that the split index
feature is incompatible with the sparse index feature, and its commit
message wondered aloud whether more checks would be required to ensure
that the split index and sparse index features aren't enabled at the
same time.

We are about to introduce such additional checks, and indeed, t1091
would utterly fail with them. Therefore, let's preemptively disable the
split index for the entirety of t1091.

This partially reverts above-mentioned patch because it covered only one
test case whereas we want to cover the entire test script.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Reviewed-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t1091-sparse-checkout-builtin.sh

@@ -5,6 +5,9 @@ test_description='sparse checkout builtin tests'
 GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME=main
 export GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME
 
+GIT_TEST_SPLIT_INDEX=false
+export GIT_TEST_SPLIT_INDEX
+
 . ./test-lib.sh
 
 list_files() {
@@ -228,36 +231,31 @@ test_expect_success 'sparse-checkout disable' '
 '
 
 test_expect_success 'sparse-index enabled and disabled' '
-	(
+	git -C repo sparse-checkout init --cone --sparse-index &&
-		sane_unset GIT_TEST_SPLIT_INDEX &&
+	test_cmp_config -C repo true index.sparse &&
-		git -C repo update-index --no-split-index &&
+	git -C repo ls-files --sparse >sparse &&
+	git -C repo sparse-checkout disable &&
+	git -C repo ls-files --sparse >full &&
 
-		git -C repo sparse-checkout init --cone --sparse-index &&
+	cat >expect <<-\EOF &&
-		test_cmp_config -C repo true index.sparse &&
+	@@ -1,4 +1,7 @@
-		git -C repo ls-files --sparse >sparse &&
+	 a
-		git -C repo sparse-checkout disable &&
+	-deep/
-		git -C repo ls-files --sparse >full &&
+	-folder1/
+	-folder2/
+	+deep/a
+	+deep/deeper1/a
+	+deep/deeper1/deepest/a
+	+deep/deeper2/a
+	+folder1/a
+	+folder2/a
+	EOF
 
-		cat >expect <<-\EOF &&
+	diff -u sparse full | tail -n +3 >actual &&
-		@@ -1,4 +1,7 @@
+	test_cmp expect actual &&
-		 a
-		-deep/
-		-folder1/
-		-folder2/
-		+deep/a
-		+deep/deeper1/a
-		+deep/deeper1/deepest/a
-		+deep/deeper2/a
-		+folder1/a
-		+folder2/a
-		EOF
 
-		diff -u sparse full | tail -n +3 >actual &&
+	git -C repo config --list >config &&
-		test_cmp expect actual &&
+	! grep index.sparse config
-
-		git -C repo config --list >config &&
-		! grep index.sparse config
-	)
 '
 
 test_expect_success 'cone mode: init and set' '