Fixes asynchronous autocompleter prefix updating

Before it could happen (if an async autocompleter returned late) that
the prefix used in the autocomplete was out of date. This fixes that.

lib/ace/autocomplete.js

@@ -197,8 +197,11 @@ var Autocomplete = function() {
                 next();
             });
         }, function() {
+            // Fetch prefix again, because they may have changed by now
+            var pos = editor.getCursorPosition();
+            var line = session.getLine(pos.row);
             callback(null, {
-                prefix: prefix,
+                prefix: util.retrievePrecedingIdentifier(line, pos.column),
                 matches: matches
             });
         });