Function truncate_diff_by_lines 

pub fn truncate_diff_by_lines(
    diff: &str,
    max_lines: usize,
    config: &CommitConfig,
) -> String

Truncate a diff to fit within a line budget, distributing lines across files by priority.

Unlike smart_truncate_diff which works on byte budgets, this operates on line counts for simpler/faster context window management in fast mode.