t1092: refactor 'sparse-index contents' test

t1092: refactor 'sparse-index contents' test

Before expanding this test with more involved cases, first extract the
repeated logic into a new test_sparse_checkout_set helper. This helper
checks that 'git sparse-checkout set ...' succeeds and then verifies
that certain directories have sparse directory entries in the sparse
index. It also verifies that the in-cone directories are _not_ sparse
directory entries in the sparse index.

Signed-off-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>