chgserver: restore pager fds attached within runcommand session

While rewriting chg in Rust, I noticed the server leaks the client's pager
fd. This isn't a problem right now since the IPC process terminates earlier
than the pager, but I believe the fds attached within a "runcommand" request
should be released as soon as the session ends.

mercurial/chgserver.py

@@ -456,7 +456,16 class chgcmdserver(commandserver.server)
         os.umask(mask)
 
     def runcommand(self):
+        # pager may be attached within the runcommand session, which should
+        # be detached at the end of the session. otherwise the pager wouldn't
+        # receive EOF.
+        globaloldios = self._oldios
+        self._oldios = []
+        try:
         return super(chgcmdserver, self).runcommand()
+        finally:
+            self._restoreio()
+            self._oldios = globaloldios
 
     def setenv(self):
         """Clear and update os.environ