VCS[git]: Add 'Revert this chunk' context menu option to diff view.

Implement in git. Add infrastructure to revert single chhunks
by using patch -R. Currently only implemented in git since
only that has functionality to re-run diff.

Rubber-stamped-by: hunger <tobias.hunger@nokia.com>

src/plugins/vcsbase/vcsbaseplugin.h

@@ -218,6 +218,10 @@ public:
                    unsigned flags = 0,
                    QTextCodec *outputCodec = 0);
 
+    // Utility to run the 'patch' command
+    static bool runPatch(const QByteArray &input, const QString &workingDirectory = QString(),
+                         int strip = 0, bool reverse = false);
+
 public slots:
     // Convenience slot for "Delete current file" action. Prompts to
     // delete the file via VCSManager.