See comments inline for details. Cc #2600.
In particular, I wanted to make sure these behaviors are not affected by #2646.
They don't seem to be.
The tests ended up weirder than expected because of
https://github.com/martinvonz/jj/issues/2600#issuecomment-1835418824. Even
though, right now, the behavior of tests is unaffected by that issue, the
*expected* behavior is different.