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/coreplugin/iversioncontrol.h

@@ -80,6 +80,14 @@ public:
 
     virtual bool managesDirectory(const QString &filename, QString *topLevel = 0) const = 0;
 
+    /*!
+     * Returns whether \a fileName is managed by this version control.
+     *
+     * \a workingDirectory is assumed to be part of a valid repository (not necessarily its
+     * top level). \a fileName is expected to be relative to workingDirectory.
+     */
+    virtual bool managesFile(const QString &workingDirectory, const QString &fileName) const = 0;
+
     /*!
      * Returns true is the VCS is configured to run.
      */