destutil: add an 'action' layer to the destmerge message dictionary We'll introduce messages for 'rebase' soon, so we introduce a way to select a message based on the action being performed.

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.