test-lib: use subshell instead of cd $new && .. && cd $old

Change the test_create_repo code added in v1.2.2~6 to use a subshell
instead of keeping track of the old working directory and cd-ing back
when it's done.

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