Before this patch, if a user is rebasing a stack of commits and
hit a conflict in between and decided to drop that commit (the commit
which is being rebased but hit conflict) and pruned it, now what
hg rebase --continue does is: skip that dropped commit and move
on to rebase the next commit and gets confused here because wdir
has two parents which is because while we skipped that dropped
commit wdir had two parents and we didn't update that to one parent.
Changes in test file demonstrate the fixed behavior.