ProjectExplorer: Show VCS topic in tree

Topic is usually the current branch name

Change-Id: Id6ecc48744a5b3d9c7502b36b4895eb18d0ff0c6
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>

src/plugins/coreplugin/iversioncontrol.h

@@ -136,6 +136,11 @@ public:
      */
     virtual QString vcsGetRepositoryURL(const QString &director) = 0;
 
+    /*!
+     * Topic (e.g. name of the current branch)
+     */
+    virtual QString vcsTopic(const QString &directory);
+
     /*!
      * Create a snapshot of the current state and return an identifier or
      * an empty string in case of failure.