Use "test_line_count" instead of "wc -l", use "git -C" instead of a
subshell, and use test_expect_code when calling difftool.  Ease
debugging by capturing output into temporary files.
Suggested-by: NJohannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: NDavid Aguilar <davvid@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>