sparse-index: complete partial expansion

sparse-index: complete partial expansion

To complete the implementation of expand_to_pattern_list(), we need to
detect when a sparse directory entry should remain sparse. This avoids a
full expansion, so we now need to use the PARTIALLY_SPARSE mode to
indicate this state.

There still are no callers to this method, but we will add one in the
next change.

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