serve: make sure to print "listening at" message immediately

If stdout is piped, status message won't be flushed until client connects to
the server and access log is written to stdout.  It seems bad idea to queue
start-up banner of server process.

mercurial/commands.py

@@ -5322,6 +5322,7 b' class httpservice(object):'
             write = self.ui.write
         write(_('listening at http://%s%s/%s (bound to %s:%d)\n') %
               (fqaddr, port, prefix, bindaddr, self.httpd.port))
+        self.ui.flush()  # avoid buffering of status message
 
     def run(self):
         self.httpd.serve_forever()