Function compute_merged_options 

fn compute_merged_options(
    current_options: &str,
    source_options: &BTreeMap<String, CmdlineOwned>,
    target_source: &SourceName,
    new_options: Option<&str>,
) -> CmdlineOwned

Compute the merged options line from all sources.

The algorithm:

1. Start with the current options line
2. Remove all options that belong to the old value of the specified source
3. Add the new options for the specified source

Options not tracked by any source are preserved as-is.