blackbox: fix exception when logging commands with format characters

When running commands like 'hg export -o mypatch-%N.patch', the blackbox
would throw an exception because it tried to format %N. This change
prevents it from trying to format the command string.

mercurial/dispatch.py

@@ -737,7 +737,7 b' def _dispatch(req):'
         ui.warn(_("warning: --repository ignored\n"))
 
     msg = ' '.join(' ' in a and repr(a) or a for a in fullargs)
-    ui.log("command", msg + "\n")
+    ui.log("command", '%s\n', msg)
     d = lambda: util.checksignature(func)(ui, *args, **cmdoptions)
     starttime = time.time()
     ret = None