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.