t1001: replace 'test -f' with 'test_path_is_file'

'test_path_is_file' is a modern path checking method in Git's development.
 Replace the basic shell command 'test -f' with this approach.

Signed-off-by: Rodrigo Carvalho <rodrigorsdc@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t1001-read-tree-m-2way.sh

@@ -363,7 +363,7 @@ test_expect_success 'a/b (untracked) vs a case setup.' '
 test_expect_success 'a/b (untracked) vs a, plus c/d case test.' '
 	read_tree_u_must_fail -u -m "$treeH" "$treeM" &&
 	git ls-files --stage &&
-	test -f a/b
+	test_path_is_file a/b
 '
 
 test_expect_success 'read-tree supports the super-prefix' '