D5279: rebase: fix two ui.logs to actually have text when using default blackbox log

[Yuya Nishihara]

> +        self.repo.ui.log("rebase",
> +                         "using in-memory rebase: %r\n" % self.inmemory,

Updated as `"...%r\n", self.inmemory`, so string formatting wouldn't be run
unless blackbox or logtoprocess is enabled.