This introduces DiffSettings.interdiff_filtering, which controls

whether the interdiff filtering logic wlil apply. At the moment, this is

intended for internal use, but we may opt to include UI for controlling

this so that users can toggle the display in the event of an issue with

interdiffs.
Interdiff filtering is always enabled by default.

Unit tests pass.
Tested this with an in-progress change for a new interdiff filtering

algorithm.