D3944: rebase: in --confirm option just abort if hit a conflict

[Yuya Nishihara]

Queued, thanks.

>                  rbsrt._prepareabortorcontinue(isabort=True, backup=False,
>                                                suppwarns=True)
>                  needsabort = False

You can remove the `if confirm:` block at all as it's handled at `finally:`.