Don't overwrite history during changes.

Fixes issues with some changes not being undone.

I mean, the sphere algorithm needs to not set blocks 20 times,
but other things can trigger this too.

Also allow radius 0 sphere via //sphere (because /br sphere allows it).

worldedit-core/src/main/java/com/sk89q/worldedit/command/GenerationCommands.java

@@ -162,13 +162,13 @@ public class GenerationCommands {
         final double radiusX, radiusY, radiusZ;
         switch (radii.size()) {
         case 1:
-            radiusX = radiusY = radiusZ = Math.max(1, radii.get(0));
+            radiusX = radiusY = radiusZ = Math.max(0, radii.get(0));
             break;
 
         case 3:
-            radiusX = Math.max(1, radii.get(0));
-            radiusY = Math.max(1, radii.get(1));
-            radiusZ = Math.max(1, radii.get(2));
+            radiusX = Math.max(0, radii.get(0));
+            radiusY = Math.max(0, radii.get(1));
+            radiusZ = Math.max(0, radii.get(2));
             break;
 
         default:
@@ -205,7 +205,7 @@ public class GenerationCommands {
                          @Arg(desc = "The density of the forest, between 0 and 100", def = "5")
                              double density) throws WorldEditException {
         checkCommandArgument(0 <= density && density <= 100, "Density must be between 0 and 100");
-        density = density / 100;
+        density /= 100;
         int affected = editSession.makeForest(session.getPlacementPosition(player), size, density, type);
         player.print(affected + " trees created.");
         return affected;

worldedit-core/src/main/java/com/sk89q/worldedit/history/changeset/BlockOptimizedHistory.java

@@ -55,7 +55,9 @@ public class BlockOptimizedHistory extends ArrayListHistory {
         if (change instanceof BlockChange) {
             BlockChange blockChange = (BlockChange) change;
             BlockVector3 position = blockChange.getPosition();
+            if (!previous.containsLocation(position)) {
                 previous.add(position, blockChange.getPrevious());
+            }
             current.add(position, blockChange.getCurrent());
         } else {
             super.add(change);