This changes the behavior in one of the cases ilyagr@
[mentioned](https://github.com/martinvonz/jj/pull/1610#discussion_r1199823932)
to match his suggestion. After some more thinking while working on
tree-level conflicts, I now think it's clear that the added `+C-C`
terms should have no effect on the result. A very similar argument is
that `Conflict::simplify()` should not change the result of
`trivial_merge()`. I'll add tests for that next.