chgserver: drop bundle.mainreporoot config

Before this patch, although chgserver drops repo object by setting it to None,
there is another side effect loading a repo: setting bundle.mainreporoot.

This patch explicitly sets it to empty to undo the side effect. It will make
chg pass test-strip.t.

hgext/chgserver.py

@@ -640,7 +640,10 b' class AutoExitMixIn: # use old-style to'
 
 class chgunixservice(commandserver.unixservice):
     def init(self):
-        self.repo = None
+        if self.repo:
+            # one chgserver can serve multiple repos. drop repo infomation
+            self.ui.setconfig('bundle', 'mainreporoot', '', 'repo')
+            self.repo = None
         self._inithashstate()
         self._checkextensions()
         class cls(AutoExitMixIn, SocketServer.ForkingMixIn,