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. It seems misleading to show "the contents of this file were not changed", because they were changed in both the sense that the file was completely removed, and also changed in the sense that the content itself was (or may have been) changed at the destination. Instead, we just show nothing.

Test Plan:

• Added test coverage.
• Ran tests.
• Used arc diff --raw --browse to verify that web behavior was consistent with CLI/test behavior.

Reviewers: joshuaspence, btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4105, T5163

Differential Revision: https://secure.phabricator.com/D11970