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.