Use relative filename in picker

[Xethron]

When trying to select a file using #file:<tab>, the picker comes up listing all files in the project with their full file name.

eg: /Users/Xethron/Code/github.com/CopilotC-Nvim/CopilotChat.nvim/README.md instead of just README.md.

This makes it hard to see deeply nested file names, especially on a smaller screen with a smaller picker.

Would be great if this could instead show relative paths in the picker, since they already work in the chat.

[kiet-ap]

yes

[lukeemhigh]

I have also a problem when accepting the full file name (i.e. the absolute path) copilot seems to not be able to see the file as context. It is only able to read the file when using the relative path.