chgserver: do not keep repo object

The current chgserver design is to use one server to handle multiple repos
which has same [extensions] config. Previously the client uses --cwd / to
avoid creating a repo object. Now we need to set repo to None before we
have code to make "serve" command norepo when it's chg.

hgext/chgserver.py

@@ -638,6 +638,7 @@ class AutoExitMixIn:  # use old-style to comply with SocketServer design
 
 class chgunixservice(commandserver.unixservice):
     def init(self):
+        self.repo = None
         self._inithashstate()
         self._checkextensions()
         class cls(AutoExitMixIn, SocketServer.ForkingMixIn,