vcs: refresh diff preview when preview editor tab is toggled on

PanelPreview has no such issue, as refresh is performed
by PreviewDiffSplitterComponent.setDetailsOn.

Fix regression after d96d3bf3f8c04162c82173ce5926522c524aaeb8

GitOrigin-RevId: 8d579ac89eeddd8ffa09fcccfda15268752a5098

platform/vcs-impl/src/com/intellij/openapi/vcs/changes/ChangesViewManager.java

@@ -740,6 +740,8 @@ public class ChangesViewManager implements ChangesViewEx,
 
       @Override
       public void setDiffPreviewVisible(boolean isVisible) {
+        updatePreview(false);
+
         if (!isVisible) {
           FileEditorManager.getInstance(myProject).closeFile(myPreviewDiffVirtualFile);
         }