unpack-trees: preserve index sparsity

When unpacking trees, set the default sparsity of the resultant index based
on repo settings and 'is_sparse_index_allowed()'.

Normally, when executing 'unpack_trees', the output index is marked sparse
when (and only when) it unpacks a sparse directory. However, an index may be
"sparse" even if it contains no sparse directories - when all files fall
inside the sparse-checkout definition or otherwise have SKIP_WORKTREE
disabled. Therefore, the output index may be marked "full" even when it is
"sparse", resulting in unnecessary 'ensure_full_index' calls when writing to
disk. Avoid this by setting the "default" index sparsity to match what is
expected for the repository.

As a consequence of this fix, the (non-merge) 'read-tree' performed when
applying a stash with untracked files no longer expands the index. Update
the corresponding test in 't1092'.

Signed-off-by: Victoria Dye <vdye@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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