perf(repeatParserService): track groups by name

Use "track by" to track groups of choices by their name instead of
the $$hastKey. This way, existing items don't have to be
re-rendered when the array of items changes.

Closes #1721

src/uisRepeatParserService.js

@@ -77,7 +77,7 @@ uis.service('uisRepeatParser', ['uiSelectMinErr','$parse', function(uiSelectMinE
   };
 
   self.getGroupNgRepeatExpression = function() {
-    return '$group in $select.groups';
+    return '$group in $select.groups track by $group.name';
   };
 
 }]);