Merge instead of rebase for restacking

Thanks for the wonderful tool! We have had a great time using it so far.

Currently, we primarily use a merge-based workflow, i.e. if there are any changes down the stack, then rather than rebasing branches upstack, we would like to merge the downstack into upstack to get these changes.

Is there a way to configure git-spice to use merges instead of rebases when restacking?