feature: picker's git_status should show staged as well

[aweis89]

Did you check the docs?

• I have read all the snacks.nvim docs

Is your feature request related to a problem? Please describe.

When running git_status with Telescope or fzf-lua, the preview shows a git diff from HEAD, so that files latest changes are viewable regardless of whether they are staged or not.
The Snacks git_status on the hand shows an empty preview window once the changes are staged.

Describe the solution you'd like

Would be great the if the git_diff previewer showed the diff from HEAD instead of just the unstaged diff.

Describe alternatives you've considered

I didn't see any configurable options for this in the source code.

Additional context

No response