rewrite: simplify the `retained_extras` extra logic First, we move the definition of value outside of the rebase extensions, as this apply to all rebase-like operation and some live in other place (like evolve). Second we make it a simple set, so that it is easy for an extension to add a new value in it. Third, we move the associated logic in core too. That make it easily available to other extensions. Fourth we simplify it usage, as the verbose version of the filtering is just a handful on line long, we are just going to test all the value for updates, so the Projection overlay is not bringing much here. Note that, we make it a module level set, is a key is worth preserving it is probably worth preserving in all cases. This was already the behavior prior to this change.