We had many places in the review request page code that called

ReviewRequest.get_latest_diffset(). Every time this was called, we

ended up performing a query, resulting in potentially dozens of these

for long review requests.
We have existing caching for ReviewRequest.get_diffsets(), which

gives the same results back for every call without additional database

queries until the local cache is reset. Since this is likely to be

called on the same page that calls get_latest_diffset(), there's value

in sharing those caches, which this change does.
The one thing that complicates that is that get_diffsets() also

pre-fetches files. This is a difference in behavior from

get_latest_diffset(), and while that can be valuable, it's in

practice an extra query across all diffsets that we don't always need.
As a solution to that, get_diffsets() now takes a with_filediffs=

argument, which get_latest_diffset() sets to False. This is designed

to be smart about its caching. If one call passes False and the next

passes True, then the first call will fetch the DiffSets and the

next will just fetch the FileDiffs using prefetch_related_objects().

If passing False when True was previously passed, it will just

simply get DiffSets with filediffs.
get_latest_diffet() does manage its own cache, but based on

get_diffsets(), optimistically avoiding a query. Detection of the

latest DiffSet now happens client-side, sorting and picking the most

recent.

All unit tests passed.
Verified this reduced the query counts on the review request page.