Fix a possible crash on shutdown while executing VcsCommand

Add a global synchronizer to the VcsPlugin.

Fixes: QTCREATORBUG-25744
Change-Id: I97578f4a5b5275071aa0253a22fc9ab2f90d9b75
Reviewed-by: Orgad Shaneh <orgads@gmail.com>

src/plugins/vcsbase/vcscommand.h

@@ -51,6 +51,9 @@ public:
                     const Utils::CommandLine &command,
                     const QString &workDirectory = {}) override;
 
+protected:
+    void addTask(QFuture<void> &future) override;
+
 private:
     void emitRepositoryChanged(const QString &workingDirectory);