t1501-worktree.sh does not copy the shared index in the "relative
$GIT_WORK_TREE and git subprocesses" test, which makes the test fail
when GIT_TEST_SPLIT_INDEX is set. Copy the shared index as well in
order to fix this.
Helped-by: Junio C Hamano <gitster@pobox.com> Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>