Git: Fix crash on show stash

* show() is called with a QString reference to the model
* And editor is opened, which triggers stateChanged()
* stateChanged() updates StashDialog, which refreshes the model with an
  empty directory (another bug)
* The QString name reference is invalidated => crash on access

Task-number: QTCREATORBUG-10397
Change-Id: I0c4077f8c4fc90905568f80d1cd89f61c4cd2cba
Reviewed-by: Eike Ziller <eike.ziller@digia.com>