t1501: fix test with split index

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: NJunio C Hamano <gitster@pobox.com>
Signed-off-by: NThomas Gummerer <t.gummerer@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

t/t1501-worktree.sh

@@ -350,6 +350,7 @@ test_expect_success 'Multi-worktree setup' '
 	mkdir work &&
 	mkdir -p repo.git/repos/foo &&
 	cp repo.git/HEAD repo.git/index repo.git/repos/foo &&
+	test_might_fail cp repo.git/sharedindex.* repo.git/repos/foo &&
 	sane_unset GIT_DIR GIT_CONFIG GIT_WORK_TREE
 '