This adds a new `revsets.simplify-parents` configuration option (similar
to `revsets.fix`) which serves as the default revset for `jj
simplify-parents` if no `--source` or `--revisions` arguments are
provided.
The previous iteration of `jj simplify-parents` would only reparent the
commits in the target set in the `MutableRepo::transform_descendants`
callback. The subsequent `MutableRepo::rebase_descendants` call invoked
when the transaction is committed would then rebase all descendants of
the target set, which includes the commits in the target set again.
This commit updates the `MutableRepo::transform_descendants` callback to
perform rebasing of all descendants within the callback. All descendants
of the target set will only be rebased at most once.
