t1501: remove use of `test_might_fail cp`

t1501: remove use of `test_might_fail cp`

The test_must_fail() family of functions (including test_might_fail())
should only be used on git commands. Replace test_might_fail() with
a compound command wrapping the old cp invocation that always returns 0.

The `test_might_fail cp` line was introduced in 466e8d5d66 (t1501: fix
test with split index, 2015-03-24). It is necessary because there might
exist some index files in `repo.git/sharedindex.*` and, if they exist,
we want to copy them over. However, if they don't exist, we don't want
to error out because we expect that possibility. As a result, we want to
keep the "might fail" semantics so we always return 0, even if the
underlying cp errors out.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>