CommitSoyData: Move last mutation of diff view builder up

Group this with other mutations so it's not hidden. This combined with
careless copy/paste may have contributed to issue 66.

Change-Id: Iba8bb381a1ffd726208672da7b179d5928549578

gitiles-servlet/src/main/java/com/google/gitiles/CommitSoyData.java

diff --git a/gitiles-servlet/src/main/java/com/google/gitiles/CommitSoyData.java b/gitiles-servlet/src/main/java/com/google/gitiles/CommitSoyData.java
index f9ca853..bba35f7 100644
--- a/gitiles-servlet/src/main/java/com/google/gitiles/CommitSoyData.java
+++ b/gitiles-servlet/src/main/java/com/google/gitiles/CommitSoyData.java
@@ -171,13 +171,15 @@
       } else {
         parentName = view.getRevision().getName() + "^" + (i++);
       }
+      diff.setOldRevision(parentName, parent);
+
       Map<String, String> e = Maps.newHashMapWithExpectedSize(4);
       e.put("sha", name);
       e.put("url", GitilesView.revision()
           .copyFrom(view)
           .setRevision(parentName, parent)
           .toUrl());
-      e.put("diffUrl", diff.setOldRevision(parentName, parent).toUrl());
+      e.put("diffUrl", diff.toUrl());
       if (fs.contains(Field.PARENT_BLAME_URL)) {
         // Assumes caller has ensured path is a file.
         e.put("blameUrl", GitilesView.blame()