sparse checkout: show error messages when worktree shaping fails

verify_* functions can queue errors up and to be printed later at
label return_failed. In case of errors, do not go to label "done"
directly because all queued messages would be dropped on the floor.

Found-by: Joshua Jensen <jjensen@workspacewhiz.com>
Tracked-down-by: Michael J Gruber <git@drmicha.warpmail.net>
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t1011-read-tree-sparse-checkout.sh

@@ -234,4 +234,19 @@ test_expect_success 'read-tree --reset removes outside worktree' '
 	test_cmp empty result
 '
 
+test_expect_success 'print errors when failed to update worktree' '
+	echo sub >.git/info/sparse-checkout &&
+	git checkout -f init &&
+	mkdir sub &&
+	touch sub/added sub/addedtoo &&
+	test_must_fail git checkout top 2>actual &&
+	cat >expected <<\EOF &&
+error: The following untracked working tree files would be overwritten by checkout:
+	sub/added
+	sub/addedtoo
+Please move or remove them before you can switch branches.
+EOF
+	test_cmp expected actual
+'
+
 test_done

unpack-trees.c

@@ -1089,6 +1089,7 @@ int unpack_trees(unsigned len, struct tree_desc *t, struct unpack_trees_options
 		 */
 		mark_new_skip_worktree(o->el, &o->result, CE_ADDED, CE_SKIP_WORKTREE | CE_NEW_SKIP_WORKTREE);
 
+		ret = 0;
 		for (i = 0; i < o->result.cache_nr; i++) {
 			struct cache_entry *ce = o->result.cache[i];
 
@@ -1101,17 +1102,23 @@ int unpack_trees(unsigned len, struct tree_desc *t, struct unpack_trees_options
 			 * correct CE_NEW_SKIP_WORKTREE
 			 */
 			if (ce->ce_flags & CE_ADDED &&
-			    verify_absent(ce, ERROR_WOULD_LOSE_UNTRACKED_OVERWRITTEN, o))
-					return -1;
+			    verify_absent(ce, ERROR_WOULD_LOSE_UNTRACKED_OVERWRITTEN, o)) {
+				if (!o->show_all_errors)
+					goto return_failed;
+				ret = -1;
+			}
 
 			if (apply_sparse_checkout(ce, o)) {
+				if (!o->show_all_errors)
+					goto return_failed;
 				ret = -1;
-				goto done;
 			}
 			if (!ce_skip_worktree(ce))
 				empty_worktree = 0;
 
 		}
+		if (ret < 0)
+			goto return_failed;
 		if (o->result.cache_nr && empty_worktree) {
 			/* dubious---why should this fail??? */
 			ret = unpack_failed(o, "Sparse checkout leaves no entry on working directory");