commandserver: close selector explicitly

The selector does not have a __del__ method and needs a manual close. We can
also use "with selector" but that makes the code too indented. Therefore
append a "selector.close()" after the end of the main loop for now.

mercurial/commandserver.py

@@ -519,6 +519,7 b' class unixforkingservice(object):'
                         self.ui.traceback(force=True)
                     finally:
                         os._exit(255)
+        selector.close()
 
     def _sigchldhandler(self, signal, frame):
         self._reapworkers(os.WNOHANG)