When gr-diff-view attaches, it may need the user's preferences in order
to set the diff mode selector to the correct state.
In the course of testing this change, I realized that an overriding
diff mode selection wasn't sticking across pages, especially when moving
back and forth in history. Modified gr-change-view to also update the
diff mode from the user's preferences when it notices a different change
is being loaded.
Bug: Issue 4434
Change-Id: Id2041b55bf66d7de66f74d7765ed0db717caeebf
78ad03c542