In a recent change (Ib83ff5157), the promise strategy for processing and
rendering diffs changed so that they resolve after all the work (i.e.
processing, diff content, and syntax highlighting) had completed.
Beforehand the promise would actually resolve after the diff processing
had finished. The diff view would wait for this promise to resolve
(along with the resolution of network calls) before removing the
`_loading` flag.
Because the promise resolution now represents all of the diff rendering
work being complete, rather than the first step only, this use of the
promise was outdated.
With this change, the diff view sets `_loading` to `false` immediately
before starting the diff processing/content/syntax pipeline.
Bug: Issue 5533
Change-Id: I24fafe4d55166e2acbf1849e6a75fbcba122b997
1333e30bd7