VCS: Clear directory cache when availability of VCSes changes

We could have gotten new VCSes or existing ones might have gone,
so clear the cache of which VCS manages which directory.

Change-Id: I87a1b9c894e2867773ea4df221e463adb14f4ee9
Reviewed-by: Petar Perisin <petar.perisin@gmail.com>
Reviewed-by: Orgad Shaneh <orgads@gmail.com>

src/plugins/coreplugin/vcsmanager.h

@@ -95,6 +95,9 @@ public:
 signals:
     void repositoryChanged(const QString &repository);
 
+public slots:
+    void clearVersionControlCache();
+
 private:
     VcsManagerPrivate *d;
 };