apply tests: don't ignore "git ls-files" exit code, drop sub-shell

apply tests: don't ignore "git ls-files" exit code, drop sub-shell

Fix code added in 969c877506c (git apply --directory broken for new
files, 2008-10-12) so that it doesn't invoke "git ls-files" on the
left-hand-side of a pipe, instead let's use an intermediate file.

Since we're doing that we can also drop the sub-shell that was here to
group the two.

There are a lot of these sorts of patterns in the test suite, and
there's no particular reason to fix this one other than in a preceding
commit all similar patterns except this one were fixed in
"t/t4128-apply-root.sh", so let's fix this one straggler as well.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>