D3829: rebase: make dry-run return 1 or 0 according to result

[Yuya Nishihara]

> --- a/hgext/rebase.py
> +++ b/hgext/rebase.py
> @@ -825,10 +825,13 @@
>                              **opts)
>          except error.InMemoryMergeConflictsError:
>              ui.status(_('hit a merge conflict\n'))
> +            retcode = 1
>          else:
> +            retcode = 0
>              ui.status(_('there will be no conflict, you can rebase\n'))
>          finally:
>              _origrebase(ui, repo, abort=True)
> +            return retcode

`retcode` may be undefined depending on the error type occurred in
`_origrebase()`. Instead, you can simply return 1 and 0 in the `except`
and `else` clauses respectively.