commandserver: do not set nontty flag if channel is replaced by a real file

This prepares for porting the chg server. In chg, a server receives client's
stdio over a UNIX domain socket to override server channels. This is because
chg should behave as if it is a normal hg command attached to tty. "nontty"
is not wanted.

This patch is corresponding to the following change. This doesn't test the
identity of "cin" object because the current version of chg reopens stdio
to apply buffering mode.

https://bitbucket.org/yuja/chg/commits/c48c7aed5fc0

mercurial/commandserver.py

@@ -215,7 +215,10 b' class server(object):'
             self.repo.invalidateall()
 
         for ui in uis:
-            # any kind of interaction must use server channels
+            # any kind of interaction must use server channels, but chg may
+            # replace channels by fully functional tty files. so nontty is
+            # enforced only if cin is a channel.
+            if not util.safehasattr(self.cin, 'fileno'):
             ui.setconfig('ui', 'nontty', 'true', 'commandserver')
 
         req = dispatch.request(args[:], copiedui, self.repo, self.cin,