It seems like I forgot to update the `jj status` output when I decided
(years ago?) that the changes in a commit should always be compared to
the auto-merged parents. I was very confused before I realized that
`jj status` was showing the diff summary against the first parent. I
suppose the fact that `jj status` lists only one parent should have
been a hint. Thanks to ilyagr@ for finding this odd behavior. This
patch fixes it by making the command list all parents, and changes the
diff summary to be against the auto-merged parents.
If this new option is not specified, we start with empty output
file and trust the merge tool did a complete merge no matter
what the file contains.
Includes tests.
