VCS: Do not try to add files that are already tracked

Task-number: QTCREATORBUG-2455
Change-Id: I4b1fa9676ad4a8d7293e035e4d6103251bef8aee
Reviewed-by: Nikita Baryshnikov <nib952051@gmail.com>
Reviewed-by: Cristian Tibirna <tibirna@kde.org>
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>

src/plugins/git/gitversioncontrol.h

@@ -48,6 +48,7 @@ public:
     Core::Id id() const;
 
     bool managesDirectory(const QString &directory, QString *topLevel) const;
+    bool managesFile(const QString &workingDirectory, const QString &fileName) const;
 
     bool isConfigured() const;
     bool supportsOperation(Operation operation) const;