diff -r c8e2d6ed1f9e -r e49c39ffeac2 mercurial/ui.py
--- a/mercurial/ui.py	Sun Jan 21 13:03:03 2018 +0900
+++ b/mercurial/ui.py	Thu Jan 25 20:00:58 2018 +0100
@@ -916,8 +916,7 @@
         # opencode timeblockedsection because this is a critical path
         starttime = util.timer()
         try:
-            for a in msgs:
-                self.fout.write(a)
+            self.fout.write(''.join(msgs))
         except IOError as err:
             raise error.StdioError(err)
         finally: