R1:c82066408d1b

Expand Differential test coverage to include moves, shields, and more

Summary:
See D11468 and D11465. Fixes T5163. Fixes T4105. This makes it practical to test shields, unshielding, moves, etc.

This fixes the issue in D11468, where line maps from whitespace-ignored hunks could have fewer lines than line maps from whitespace-respected hunks, causing a warning.

This encodes the behavior which D11465 changed, making it the canon behavior. Specifically, we do **not** show a shield. I think this is correct.