VCS: Add id to IVersionControl

... and all derived classes

src/plugins/mercurial/mercurialcontrol.h

@@ -53,6 +53,7 @@ public:
     explicit MercurialControl(MercurialClient *mercurialClient);
 
     QString displayName() const;
+    QString id() const;
     bool managesDirectory(const QString &filename, QString *topLevel = 0) const;
     bool isConfigured() const;
     bool supportsOperation(Operation operation) const;