t0000: use test_path_is_file instead of "test -f"

Signed-off-by: Caleb Tillman <caleb.tillman@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t0000-basic.sh

@@ -1191,7 +1191,7 @@ test_expect_success 'writing this tree with --missing-ok' '
 test_expect_success 'git read-tree followed by write-tree should be idempotent' '
 	rm -f .git/index &&
 	git read-tree $tree &&
-	test -f .git/index &&
+	test_path_is_file .git/index &&
 	newtree=$(git write-tree) &&
 	test "$newtree" = "$tree"
 '