Only do the delete hack if it's the first key press

This avoids unexpectedly deleting when you spam backspace
to clear a failed search query.

index.html

@@ -27,6 +27,7 @@
         <script src='js/lib/d3.trigger.js'></script>
         <script src='js/lib/d3.keybinding.js'></script>
         <script src='js/lib/d3.clip.js'></script>
+        <script src='js/lib/d3.one.js'></script>
         <script src='js/lib/d3-compat.js'></script>
         <script src='js/lib/bootstrap-tooltip.js'></script>
         <script src='js/lib/rtree.js'></script>

js/id/ui/preset_grid.js

@@ -26,7 +26,7 @@ iD.ui.PresetGrid = function(context) {
         var search = searchwrap.append('input')
             .attr('class', 'preset-grid-search')
             .attr('type', 'search')
-            .on('keydown', function() {
+            .one('keydown', function() {
                 // hack to let delete shortcut work when search is autofocused
                 if (search.property('value').length === 0 &&
                     (d3.event.keyCode === d3.keybinding.keyCodes['⌫'] ||

js/lib/d3.one.js

@@ -5,4 +5,5 @@ d3.selection.prototype.one = function (type, listener, capture) {
         listener.apply(this, arguments);
     }
     target.on(typeOnce, one, capture);
+    return this;
 };