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

[Yuya Nishihara]

> -            retcode = rbsrt._prepareabortorcontinue(abortf)
> +            # If in-memory, means aborting during dry-run, no need to backup
> +            backup = not rbsrt.inmemory
> +            retcode = rbsrt._prepareabortorcontinue(abortf, backup=backup)

This seems confusing and is probably wrong since we wouldn't overwrite
`inmemory` to `False` if in-memory rebase were resumable.

I think explicit `backup` flag is less bad.