DiffEditor: Fix build

diffutils.cpp: In function 'QList<DiffEditor::RowData>
    DiffEditor::readLines(QStringRef, bool, bool*, bool*)':
diffutils.cpp:585:53: error: no match for 'operator+' (operand types are
    'QStringRef' and 'const QChar')

Change-Id: I44ad2237c033266d3832ce95174ec056ff7cae9f
Reviewed-by: Orgad Shaneh <orgads@gmail.com>

src/plugins/diffeditor/diffutils.cpp

@@ -582,7 +582,7 @@ static QList<RowData> readLines(QStringRef patch,
                 break;
             }
 
-            Diff diffToBeAdded(command, line.mid(1) + newLine);
+            Diff diffToBeAdded(command, line.mid(1).toString() + newLine);
 
             if (!diffList.isEmpty() && diffList.last().command == command)
                 diffList.last().text.append(diffToBeAdded.text);