rebase: extract rebaseset and destination computation in a function The whole rebase function is gargantuan and this computation is almost 100 lines long. We extract it in a dedicated function as it is independent from the rest of the rebase code. Having it in its own function will make it easier to rework the default destination logic.

tests: have a more elaborated test for _destrebase The previous repo had a single lineage and a single head, pretty boring for rebase related testing.