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.
I don't know why I made it return an owned value. It seems like an
unnecessary restriction that the value implements `Clone`, so let's
return a reference instead.
We already resolve merge conflicts between hunks, trees, and refs, and
maybe more. They each have their own code for the handling trivial
merges (where the output is equal to one of the inputs). They look
surprisingly different. This commit adds a generic function for doing
that. Curiously, this new implementation uses implements it in yet
another way (basically using a multi-set).
