name-hash: use expand_to_path()

name-hash: use expand_to_path()

A sparse-index loads the name-hash data for its entries, including the
sparse-directory entries. If a caller asks for a path that is contained
within a sparse-directory entry, we need to expand to a full index and
recalculate the name hash table before returning the result. Insert
calls to expand_to_path() to protect against this case.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Reviewed-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>