util.js: Fix prefix_sort logic to not mutate input.

In prefix sort, shifting of objs list to iterate through the elements
caused the 'emoji_show_list' to be emptied each time it was passed as
argument for sorting.

This modifies prefix sort to prevent it from modifying the objs list passed
as argument - changed it to normal iteration rather than popping
the elements from objs list.

static/js/util.js

@@ -241,10 +241,10 @@ exports.prefix_sort = function (query, objs, get_item) {
     var beginswithCaseSensitive = [];
     var beginswithCaseInsensitive = [];
     var noMatch = [];
-
+    var obj;
-    var obj = objs.shift();
-    while (obj) {
     var item;
+    for (var i = 0; i < objs.length; i += 1) {
+        obj = objs[i];
         if (get_item) {
             item = get_item(obj);
         } else {
@@ -257,7 +257,6 @@ exports.prefix_sort = function (query, objs, get_item) {
         } else {
             noMatch.push(obj);
         }
-        obj = objs.shift();
     }
     return { matches: beginswithCaseSensitive.concat(beginswithCaseInsensitive),
              rest:    noMatch };