ui: improve performance for multi-component writes

It is more efficient to pass down one large string to the output streams
than many small ones. For a ``hg diff`` test case, it improves time from
2m26s to 2m8s.

Differential Revision: https://phab.mercurial-scm.org/D1939

mercurial/ui.py

@@ -916,8 +916,7 b' class ui(object):'
         # 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: