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.
ERROR: FIXME: This is a known failure of filelog.size for data starting with \1\n
OK.