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

Some implementations of ui.log record structured information along with the
ui.log which can be used for metrics, but ui.log() as implemented by the
blackbox logging does not do anything special with this, and we end up with a
log line with no text (not even a line break) so it ends up looking something
like:

    date time user @node (pid) [rebase]> date time user @node (pid) ...

Differential Revision: https://phab.mercurial-scm.org/D5279

hgext/rebase.py

@@ -404,7 +404,9 b' class rebaseruntime(object):'
         else:
             self.wctx = self.repo[None]
             self.repo.ui.debug("rebasing on disk\n")
-        self.repo.ui.log("rebase", "", rebase_imm_used=self.inmemory)
+        self.repo.ui.log("rebase",
+                         "using in-memory rebase: %r\n", self.inmemory,
+                         rebase_imm_used=self.inmemory)
 
     def _performrebase(self, tr):
         self._assignworkingcopy()
@@ -1076,7 +1078,8 b' def _definedestmap(ui, repo, inmemory, d'
             return None
 
     rebasingwcp = repo['.'].rev() in rebaseset
-    ui.log("rebase", "", rebase_rebasing_wcp=rebasingwcp)
+    ui.log("rebase", "rebasing working copy parent: %r\n", rebasingwcp,
+           rebase_rebasing_wcp=rebasingwcp)
     if inmemory and rebasingwcp:
         # Check these since we did not before.
         cmdutil.checkunfinished(repo)