Previously, when a diff was reloaded, it would clear the diff content
(the previous diff) and open requests for the data regarding the new
diff. When the requests finished, it would cancel async processing and
start it again on the new data. In this arrangement, a narrow race
condition was present where the async rendering could emit a diff from
the previous diff after the diff table had been cleared, but before the
network requests resolved -- resulting is content form the previous diff
appearing in the new one.
With this change, the async processing is cancelled at the same time
that the diff content is cleared.
Bug: Issue 6170
Change-Id: Ie5a807082ab51156595f9eeb9ba6c958d41e890a
21e6b7f207