t6402: preserve git exit status code

t6402: preserve git exit status code

In t6402, we're checking number of files in the index and the working
tree by piping the output of Git's command to "wc -l", thus losing the
exit status code of git.

Let's use the new helper test_stdout_line_count in order to preserve
Git's exit status code.

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