DiffEditor: fix revert chunk for diffs of modified files

Apply chunk action doesn't make sense for that case,
will stay disabled.

Task-number: QTCREATORBUG-17136
Change-Id: Idce31b3aa9d354536a01607c10b20273158961d5
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>

src/plugins/texteditor/textdocument.h

@@ -118,6 +118,7 @@ public:
     OpenResult open(QString *errorString, const QString &fileName,
                     const QString &realFileName) override;
     virtual bool reload(QString *errorString);
+    bool reload(QString *errorString, const QString &realFileName);
 
     bool setPlainText(const QString &text);
     QTextDocument *document() const;