test-lib-functions: introduce test_stdout_line_count

test-lib-functions: introduce test_stdout_line_count

In some tests, we're checking the number of lines in output of some
commands, including but not limited to Git's command.

We're doing the check by running those commands in the left side of
a pipe, thus losing the exit status code of those commands. Meanwhile,
we really want to check the exit status code of Git's command.

Let's write the output of those commands to a temporary file, and use
test_line_count separately in order to check exit status code of
those commands properly.

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