Allow to chose the fallback version in case of conflict when performing merge

In case of conflict, the default behaviour right now is to always fallback on the current version: this means that there is always a result in case of conflict, and this result is always the current version.
It would be helpful to be able to configure the fallback version, to allow getting the next version in fallback: then API consumers would be able to chose solving conflicts with the next version just by setting that config.