Fixes internal bug #762964

Rename tracking usually triggers the TrackingSessionUpdated event
whenever the buffer changes, except for buffer changes that occur before
the _isRenamableIdentifierTask completes. Prior to this change, rename
tracking would get stuck in its initial state between the completion of
_isRenamableIdentifierTask and the next buffer change. We now also
update the session when _isRenamableIdentifierTask completes.