feat(review): add "All files" diff type

[backnotprop]

Summary

• Adds an "All files" diff type that diffs the empty tree against HEAD, showing every tracked file as an addition
• Solves the case where a repo has only committed changes (no working tree diffs) and there's no way to launch a review
• Available in the diff type dropdown, as a default preference, and works with worktrees

Test plan

• Open a review on a repo with no uncommitted/staged/unstaged changes — select "All files" from the dropdown
• Verify every tracked file appears as additions
• Verify file expansion (click to expand context) works correctly
• Verify "All files" works in a worktree
• Verify it can be set as the default diff type via the setup dialog or config