t6400: preserve git ls-files exit status code

In t6400, we're checking number of files in the index and the working
tree by piping the output of "git ls-files" to "wc -l", thus losing the
exit status code of git.

Let's use the newly introduced test_stdout_line_count in order to check
the exit status code of Git's command.

Signed-off-by: Đoàn Trần Công Danh <congdanhqx@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
