commandserver: drop tell() and seek() from channels (issue5049)

These operations are obviously invalid for file-like channels because they
will read or write protocol headers.

This patch works around the issue that "hg archive" generates a corrupted
zip file on Windows commandserver because of unusable tell() implementation.
But the problem still occurs without using a commandserver.

  $ hg archive -R not-small-repo -t zip - | cat > invalid.zip

So, this patch cannot fix the issue5049 completely.

mercurial/commandserver.py

@@ -55,7 +55,7 b' class channeledoutput(object):'
         self.out.flush()
 
     def __getattr__(self, attr):
-        if attr in ('isatty', 'fileno'):
+        if attr in ('isatty', 'fileno', 'tell', 'seek'):
             raise AttributeError(attr)
         return getattr(self.out, attr)
 
@@ -139,7 +139,7 b' class channeledinput(object):'
         return l
 
     def __getattr__(self, attr):
-        if attr in ('isatty', 'fileno'):
+        if attr in ('isatty', 'fileno', 'tell', 'seek'):
             raise AttributeError(attr)
         return getattr(self.in_, attr)