DiffFragmentView and the main subclass, ReviewsDiffFragmentView,

performed a number of expensive operations prior to the point where we

could generate an ETag. When we did reach that point, we generated an

ETag based off the cache key, which needed a lot of information that was

otherwise available in the URL. On top of all this, we stored a lot of

state (such as FileDiffs) on the instance, making it hard to extend this

class with a version that could generate fragments for multiple

FileDiffs.
Now, a lot more of the work happens in the base DiffFragmentView. The

responsibilities of the different functions are a bit more clear (for

instance, create_renderer is no longer trying to query DiffSet/FileDiff

instances). ReviewsDiffFragmentView has a lot less it must override, and

what it does need to override is a lot more controlled.
The end result is a more organized view that can perform very fast ETag

checks with very fast responses, bringing the total request/response

time for cached pages into the single-digit millisecond range (at least

locally -- probably double digits otherwise).
Some future work will build upon the reorganization to let us stream

diff fragments in a single HTTP request, avoiding a lot of connection

and database query overhead.

Tested with a bunch of diffs.
Unit tests pass.
I checked that 304s were very quickly returned with minimum code execution

if the diff was browsed before.