TrackMatchMode: arms are now treated as separate-but-equal (in end effect, that is).

This is to handle a case like:

    match s {
        Err(_) => s,
        Ok(content) => x(content),
    }

which moves on the `Ok` arm pattern but not on the `Err` arm pattern,
and this is okay because it ends up moving the `s` on both *paths*
(eventually).
root testsuite: separate-but-equal treatment for match arms.
root testsuite: better doc for what foo19.rs is doing.