sparse-index: include EXTENDED flag when expanding

sparse-index: include EXTENDED flag when expanding

When creating a full index from a sparse one, we create cache entries
for every blob within a given sparse directory entry. These are
correctly marked with the CE_SKIP_WORKTREE flag, but the CE_EXTENDED
flag is not included. The CE_EXTENDED flag would exist if we loaded a
full index from disk with these entries marked with CE_SKIP_WORKTREE, so
we can add the flag here to be consistent. This allows us to directly
compare the flags present in cache entries when testing the sparse-index
feature, but has no significance to its correctness in the user-facing
functionality.

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