XMLShapeExport::ImpExportGraphicObjectShape() unnecessarily swaps out
the GraphicObject by calling setPropertyValue("GraphicStreamURL")
even if the URL didn't actually change from what was retrieved
just a couple lines earlier, incidentally swapping it in too.

Well actually it isn't really swapped out, it's marked as auto-swapped,
but nevertheless on getting the "ReplacementGraphicURL" property
its Graphic will be replaced by swapping it in again.

So don't do that, then it's only swapped in once.

This speeds up round-tripping the ML bugdoc from 3:20 to 3:00.

Change-Id: I65a211a0c225444c06d5516df9c6716360be46c0