i3-save-tree: only create "swallows" key for leaf nodes (Thanks xeen)

fixes #1149

i3-save-tree

@@ -1,7 +1,7 @@
 #!/usr/bin/env perl
 # vim:ts=4:sw=4:expandtab
 #
-# © 2013 Michael Stapelberg
+# © 2013-2014 Michael Stapelberg
 #
 # Requires perl ≥ v5.10, AnyEvent::I3 and JSON::XS
 
@@ -149,12 +149,15 @@ sub dump_containers {
         say "$ws// $desc with " . @{$tree->{nodes}} . " children";
     }
 
-    # Turn “window_properties” into “swallows” expressions.
+    # Turn “window_properties” into “swallows” expressions, but only for leaf
+    # nodes. It only makes sense for leaf nodes to swallow anything.
+    if (leaf_node($tree)) {
         my $swallows = {};
         for my $property (keys %{$tree->{window_properties}}) {
             $swallows->{$property} = '^' . quotemeta($tree->{window_properties}->{$property}) . '$';
         }
         $tree->{swallows} = [ $swallows ];
+    }
     delete $tree->{window_properties};
 
     my @keys = sort keys %$tree;