name-hash: don't add sparse directories in threaded lazy init

name-hash: don't add sparse directories in threaded lazy init

Ensure that logic added in 5f11669586 (name-hash: don't add directories
to name_hash, 2021-04-12) also applies in multithreaded hashtable init
path.

As per the original single-threaded change above: sparse directory entries
represent a directory that is outside the sparse-checkout definition.
These are not paths to blobs, so should not be added to the name_hash
table. Instead, they should be added to the directory hashtable when
'ignore_case' is true.

Add a condition to avoid placing sparse directories into the name_hash
hashtable. This avoids filling the table with extra entries that will
never be queried.

Signed-off-by: Alex Mironov <alexandrfox@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>