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>