D3827: rebase: no need to store backup during dry-run while aborting

[Yuya Nishihara]

> -    def _prepareabortorcontinue(self, isabort):
> +    def _prepareabortorcontinue(self, isabort, opts={}):

Mutable default value should be avoided. It can be `backup=True` since we
aren't processing command-level thingy here.

And please make sure tests pass before sending out patches.

> @@ -828,7 +829,8 @@
>          else:
>              ui.status(_('there will be no conflict, you can rebase\n'))
>          finally:
> -            _origrebase(ui, repo, abort=True)
> +            opts = {'abort':True, 'no_backup':True}
> +            _origrebase(ui, repo, **opts)

Maybe we can refactor the dryrun handling in a way we don't have to pass around
the no_backup flag. IIUC, `_origrebase()` is a high-level function which has
many parameter/state checks, but what we want is to cancel the in-memory
session we've made just before. No user error should occur.

> @@ -1588,7 +1590,10 @@
>  
>              # Strip from the first rebased revision
>              if rebased:
> -                repair.strip(repo.ui, repo, strippoints)
> +                if nobackup:
> +                    repair.strip(repo.ui, repo, strippoints, backup=False)
> +                else:
> +                    repair.strip(repo.ui, repo, strippoints)

This can be written as `backup=not nobackup` or `backup=backup`.