D3870: rebase: add --confirm option

[Yuya Nishihara]

Queued, thanks.

> -    ui.status(_('starting dry-run rebase; repository will not be changed\n'))
> +    confirm = opts.get('confirm')
> +    if confirm:
> +        ui.status(_('starting rebase...\n'))

Nit: this message is misleading since nothing will be committed until
accepting the changes. Can you remove it? Or alternatively, we can say that
you're doing rebase in-memory so you're safe.

>      with repo.wlock(), repo.lock():
>          try:
> +            needsabort = True

Moved this before the "try".

>          except error.InMemoryMergeConflictsError:
>              ui.status(_('hit a merge conflict\n'))
> +            if confirm:
> +                # abort as in-memory merge doesn't support conflict
> +                rbsrt._prepareabortorcontinue(isabort=True, backup=False,
> +                                              suppwarns=True)
> +                needsabort = False
> +                if not ui.promptchoice(_(b'apply changes (yn)?'
> +                                         b'$$ &Yes $$ &No')):

Nit: This isn't actually "apply changes".

> +                    _dorebase(ui, repo, opts, inmemory=False)

`return _dorebase(...)` to not loose the actual result code. Can you
fix by a follow-up patch?