t7063: use --force-untracked-cache to speed up a bit

When in the middle of t7063, we are sure untracked cache is supported,
so we can use --force-untracked-cache to skip the support detection
phase and save a few seconds. It's also good that --force-untracked-cache
is exercised in the test suite.

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

t/t7063-status-untracked-cache.sh

@@ -402,7 +402,7 @@ test_expect_success 'set up sparse checkout' '
 	echo "done/[a-z]*" >.git/info/sparse-checkout &&
 	test_config core.sparsecheckout true &&
 	git checkout master &&
-	git update-index --untracked-cache &&
+	git update-index --force-untracked-cache &&
 	git status --porcelain >/dev/null && # prime the cache
 	test_path_is_missing done/.gitignore &&
 	test_path_is_file done/one