This fixes a bug I've run into somewhat frequently. What happens is
that if you have a conflict on top of another conflict and you resolve
the conflict in the bottom commit, we just simplify the `Conflict`
object in the second commit, but we don't try to resolve the new
conflict. That shows up as an unexpected "conflict" in `jj log`
output, and when you check out the commit, there are actually no
conflicts, so you can just `jj squash` right away.
This patch fixes that bug. It also teaches the code to work with more
than 3 parts in the merge, so if there's a 5-way conflict, for
example, we still try to resolve it if possible.
c0ae4b16e8