Git: Fix potential heap use after free

Closing the dialog before the process terminates might lead to this.

Task-number: QTCREATORBUG-13075
Change-Id: I3b6ffcca010eb356b14d87f2a7d62090c158faba
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@digia.com>
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>

src/plugins/git/changeselectiondialog.h

@@ -85,6 +85,7 @@ private slots:
 
 private:
     void enableButtons(bool b);
+    void terminateProcess();
 
     Ui::ChangeSelectionDialog *m_ui;