tdf#99386, broadcast that cell content changed after merge cells undo/redo

Change-Id: Idf41d1b07e26af66d9eefd18e5d5b582edc205e9
Reviewed-on: https://gerrit.libreoffice.org/29370Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Markus Mohrhard <markus.mohrhard@googlemail.com>

sc/source/ui/undo/undoblk3.cxx

@@ -727,6 +727,8 @@ void ScUndoMerge::DoChange( bool bUndo ) const
 
         if (!bDidPaint)
             ScUndoUtil::PaintMore(pDocShell, aRange);
+
+        rDoc.BroadcastCells(aRange, SC_HINT_DATACHANGED);
     }
 
     ShowTable(aCurRange);