/diff display broken for new files

[FedeArre]

Describe the bug

When trying to visualize the diff of a new file, the visualization is fully broken, displaying only 1 line (censored on red, is just a random line from the file).

Affected version

GitHub Copilot CLI 0.0.411.

Steps to reproduce the behavior

1. Have the agent create a new file
2. Use /diff

Expected behavior

The diff should work fine, showing the whole file green (Because is new)

Additional context

Windows 11 (x64), Powershell.