Function tab_completion_target 

pub fn tab_completion_target(
    buffer: &str,
    matches: &[&CommandSpec],
) -> Option<String>

Return the canonical name to autocomplete to, given the current buffer and the set of matches.

Algorithm: zero matches yields None; exactly one match yields Some(canonical_name); multiple matches yield the longest common prefix of all canonical names, but only if it strictly extends the buffer (otherwise None, leaving the buffer alone).