ChangeScreen2: Make diff against revision persistent

2d2a3c405e added diff against to the change
screen2. However base revision is lost when side by side view is closed.

This change fixes that.
Change-Id: I7f74760fde96469a60556f657923a3936d6516c7

gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/Header.java

@@ -81,6 +81,7 @@ class Header extends Composite {
     SafeHtml.setInnerHTML(filePath, formatPath(path));
     up.setTargetHistoryToken(PageLinks.toChange(
         patchSetId.getParentKey(),
+        base != null ? String.valueOf(base.get()) : null,
         String.valueOf(patchSetId.get())));
   }