Merge branch 'en/sparse-checkout'

The behaviour of "sparse-checkout" in the state "git clone
--no-checkout" left was changed accidentally in 2.27, which has
been corrected.

* en/sparse-checkout:
  sparse-checkout: avoid staging deletions of all files

builtin/sparse-checkout.c

@@ -99,6 +99,10 @@ static int update_working_directory(struct pattern_list *pl)
 	struct lock_file lock_file = LOCK_INIT;
 	struct repository *r = the_repository;
 
+	/* If no branch has been checked out, there are no updates to make. */
+	if (is_index_unborn(r->index))
+		return UPDATE_SPARSITY_SUCCESS;
+
 	memset(&o, 0, sizeof(o));
 	o.verbose_update = isatty(2);
 	o.update = 1;

t/t1091-sparse-checkout-builtin.sh

@@ -100,6 +100,28 @@ test_expect_success 'clone --sparse' '
 	check_files clone a
 '
 
+test_expect_success 'interaction with clone --no-checkout (unborn index)' '
+	git clone --no-checkout "file://$(pwd)/repo" clone_no_checkout &&
+	git -C clone_no_checkout sparse-checkout init --cone &&
+	git -C clone_no_checkout sparse-checkout set folder1 &&
+
+	git -C clone_no_checkout sparse-checkout list >actual &&
+	cat >expect <<-\EOF &&
+	folder1
+	EOF
+	test_cmp expect actual &&
+
+	# nothing checked out, expect "No such file or directory"
+	! ls clone_no_checkout/* >actual &&
+	test_must_be_empty actual &&
+	test_path_is_missing clone_no_checkout/.git/index &&
+
+	# No branch is checked out until we manually switch to one
+	git -C clone_no_checkout switch master &&
+	test_path_is_file clone_no_checkout/.git/index &&
+	check_files clone_no_checkout a folder1
+'
+
 test_expect_success 'set enables config' '
 	git init empty-config &&
 	(