When collecting FileDiffs in DiffSet.per_commit_files() and

DiffSet.cumulative_files(), it was possible to get files that weren't

in an expected order. These could come from the database in index order

and not insertion order. While this generally wouldn't be a huge

problem for general lookups of files (and wasn't a problem for all

database setups), it could lead to out-of-order raw diffs, which could

easily break anything trying to access them.
This is an edge case for sure, and one where we don't have a good

reproduction case. It seems to be dependent on the database.
We now explicitly force an order based on the primary key. Currently,

we're only enforcing them in these properties, and not as the default

sort order for the model, in order to avoid any unintentional side

effects.

Unit tests pass.
Tested in production with a customer who encountered this problem. We

verified the fix.