t2104: make sure split index mode is off for the version test

Version tests only make sense when all entries are in the same file,
so we can see if version is downgraded to 2 if 3 is not required.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t2104-update-index-skip-worktree.sh

@@ -7,6 +7,8 @@ test_description='skip-worktree bit test'
 
 . ./test-lib.sh
 
+sane_unset GIT_TEST_SPLIT_INDEX
+
 test_set_index_version 3
 
 cat >expect.full <<EOF