1. I had changed the preview pane service to just return the preview object that was passed in in cases where we have no additional information to display (e.g. in the case of refactorings, there is no diagnostic and so there is no need to add a diagnostic header). Unfortunately, the passed in object (IWpfDifferenceViewer) is not a wpf control that the platform can display - so this change ended up causing us to stop showing a preview in such cases. I changed the code back to do what it was doing before my change (i.e. get the visual control from from the passed in IWpfDifferenceViewer and wrap that with the preview pane). We'll need this later anyways once refactorings get headers...

2. In cases where header was absent, we were still displaying a spurious separator between non-existent header and the preview diff view. Changed the code to only display the separator when required. (changeset 1411804)