fix: legacy diff options not working due to merge order

[ddzero2c]

Summary

Fixes a bug where the legacy diff options vertical_split and open_in_current_tab were not working correctly.

Problem

The backward compatibility mapping was checking if fields were nil after merging with defaults, but those fields were already populated by the merge, so the legacy options were never applied.

Example of broken behavior:

require("claudecode").setup({
  diff_opts = {
    vertical_split = false,  -- This was ignored! Always got vertical split
  }
})

Solution

Changed the logic to unconditionally apply legacy option mappings when the legacy options are present. This ensures backward compatibility without complex conditional checks.

Changes:

• vertical_split=false now correctly sets layout="horizontal"
• vertical_split=true now correctly sets layout="vertical"
• open_in_current_tab=false now correctly sets open_in_new_tab=true
• open_in_current_tab=true now correctly sets open_in_new_tab=false

Testing

Manually tested all scenarios:

• ✅ Legacy options work correctly
• ✅ New options continue to work
• ✅ Default behavior unchanged
• ✅ make format passes (no changes needed)

🤖 Generated with Claude Code