At some point, I tried `new_commit` instead of `rewrite_commit` in the split
command. That seemed to work, but messed up the dates in a subtle way.
This commit should prevents repeats of this mistake and emphasize the
importance of the author dates being preserved.
One use case for `jj split` is when creating a new commit from some of
the changes in the working copy. If there's no description on the
working-copy commit in that case, it seems better to not ask the user
to provide one when they're splitting the commit either.
