Vcs: Add variables containing the Vcs name (Git, etc.) and topic

This allows to have the git branch names (or tags, etc.) in the
build path or to pass that information to scripts.

Task-number: QTCREATORBUG-10376
Change-Id: I151990bdc7b85abbb427b2afae220adb4e918f4f
Reviewed-by: Eike Ziller <eike.ziller@digia.com>

src/plugins/vcsbase/vcsplugin.h

@@ -72,6 +72,7 @@ signals:
 
 private slots:
     void slotSettingsChanged();
+    void updateVariable(const QByteArray &variable);
 
 private:
     void populateNickNameModel();