sparse checkout: inhibit empty worktree

The way sparse checkout works, users may empty their worktree
completely, because of non-matching sparse-checkout spec, or empty
spec. I believe this is not desired. This patch makes Git refuse to
produce such worktree.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t1009-read-tree-sparse-checkout.sh

@@ -55,20 +55,16 @@ test_expect_success 'read-tree --no-sparse-checkout with empty .git/info/sparse-
 	test -f sub/added
 '
 
-cat >expected.swt <<EOF
-S init.t
-S sub/added
-EOF
 test_expect_success 'read-tree with empty .git/info/sparse-checkout' '
 	git config core.sparsecheckout true &&
 	echo > .git/info/sparse-checkout &&
-	git read-tree -m -u HEAD &&
+	test_must_fail git read-tree -m -u HEAD &&
 	git ls-files --stage > result &&
 	test_cmp expected result &&
 	git ls-files -t > result &&
 	test_cmp expected.swt result &&
-	test ! -f init.t &&
-	test ! -f sub/added
+	test -f init.t &&
+	test -f sub/added
 '
 
 cat >expected.swt <<EOF

unpack-trees.c

@@ -498,6 +498,7 @@ int unpack_trees(unsigned len, struct tree_desc *t, struct unpack_trees_options
 	}
 
 	if (!o->skip_sparse_checkout) {
+		int empty_worktree = 1;
 		for (i = 0;i < o->result.cache_nr;i++) {
 			struct cache_entry *ce = o->result.cache[i];
 
@@ -512,8 +513,14 @@ int unpack_trees(unsigned len, struct tree_desc *t, struct unpack_trees_options
 			 */
 			if (ce_skip_worktree(ce))
 				ce->ce_flags &= ~(CE_UPDATE | CE_REMOVE);
+			else
+				empty_worktree = 0;
 
 		}
+		if (o->result.cache_nr && empty_worktree) {
+			ret = unpack_failed(o, "Sparse checkout leaves no entry on working directory");
+			goto done;
+		}
 	}
 
 	o->src_index = NULL;