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>

t/t1092-sparse-checkout-compatibility.sh

@@ -1392,7 +1392,7 @@ test_expect_success 'sparse-index is not expanded: stash' '
 	ensure_not_expanded stash -u &&
 	(
 		WITHOUT_UNTRACKED_TXT=1 &&
-		! ensure_not_expanded stash pop
+		ensure_not_expanded stash pop
 	) &&
 
 	ensure_not_expanded stash create &&

unpack-trees.c

@@ -11,6 +11,7 @@
 #include "refs.h"
 #include "attr.h"
 #include "split-index.h"
+#include "sparse-index.h"
 #include "submodule.h"
 #include "submodule-config.h"
 #include "fsmonitor.h"
@@ -1839,6 +1840,11 @@ int unpack_trees(unsigned len, struct tree_desc *t, struct unpack_trees_options
 	o->result.fsmonitor_last_update =
 		xstrdup_or_null(o->src_index->fsmonitor_last_update);
 
+	if (!o->src_index->initialized &&
+	    !repo->settings.command_requires_full_index &&
+	    is_sparse_index_allowed(&o->result, 0))
+		o->result.sparse_index = 1;
+
 	/*
 	 * Sparse checkout loop #1: set NEW_SKIP_WORKTREE on existing entries
 	 */