Function slice_hunk_with_state 

pub fn slice_hunk_with_state(
    hunk: &DiffHunk,
    picked: &[bool],
    selected: &[bool],
) -> Result<DiffHunk>

Slice a hunk using picked/selected state masks (for split command).

This builds a patch that correctly accounts for previously picked lines:

• ‘+’ lines: selected -> keep, already picked -> context, else drop
• ‘-’ lines: selected -> keep, already picked -> drop, else context
• context: always keep

Both picked and selected are masks over hunk.lines (same length).