destutil: remove current head from list of candidates early While 'hg merge' will refuse to pick a default destination if the working copy is not on a head, this will be a common and valid case for rebase. In this case, we will need to exclude from the candidate destination all descendants from the rebased set. We make a step forward in that direction by removing parents of the working copy from the candidate destinations instead of manually filtering the working copy parent at the end of the process. This will make the extra step of filtering descendant much simpler in a future changeset.

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.

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.