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.cpp

@@ -41,4 +41,9 @@ QString IVersionControl::vcsMakeWritableText() const
     return QString();
 }
 
+QString IVersionControl::vcsTopic(const QString &)
+{
+    return QString();
+}
+
 }