restack: optional conflict resolution

Right now, when we restack, users must resolve all conflicts at restack time.
This is a good default, but it's not always convenient.

An improvement here can be made for multi-branch workflows:
if there's a conflict, present the user an option to pick whether they want to
resolve the conflict now or later.
If they pick now, we let them fix it and run `gs rebase continue`.
Otherwise, we skip that branch and its upstack from the restack operation,
and print a list of branches that were skipped for them to fix manually later.