gr-diff keeps track of key locations and whenever it renders, does not
collapse code in those locations even if the diff is unchanged.
There is code to update those keylocations whenever comments are added
or removed, however, unfortunately the node observer for the new
comments fires *after* rendering, so the comments were not taken into
account. This change synchronously checks for any comments just before
rendering instead.
Change-Id: Ia46e15ffba4b6f702b6215c61a4774c2e60b0bf5
79efa8a328