VCS: Add id to IVersionControl

... and all derived classes

src/plugins/git/gitversioncontrol.cpp

@@ -36,6 +36,7 @@
 #include "gitutils.h"
 
 #include <utils/qtcassert.h>
+#include <vcsbase/vcsbaseconstants.h>
 
 #include <QtCore/QDebug>
 #include <QtCore/QFileInfo>
@@ -56,6 +57,11 @@ QString GitVersionControl::displayName() const
     return QLatin1String("git");
 }
 
+QString GitVersionControl::id() const
+{
+    return QLatin1String(VCSBase::Constants::VCS_ID_GIT);
+}
+
 bool GitVersionControl::isConfigured() const
 {
     bool ok = false;