Use /dev/null for update hook stdin.

Use /dev/null for update hook stdin.

Currently the update hook invoked by receive-pack has its stdin
connected to the pushing client.  The hook shouldn't attempt to
read from this stream, and doing so may consume data that was
meant for receive-pack.  Instead we should give the update hook
/dev/null as its stdin, ensuring that it always receives EOF and
doesn't disrupt the protocol if it attempts to read any data.

The post-update hook is similar, as it gets invoked with /dev/null
on stdin to prevent the hook from reading data from the client.
Previously we had invoked it with stdout also connected to /dev/null,
throwing away anything on stdout, to prevent client protocol errors.
Instead we should redirect stdout to stderr, like we do with the
update hook.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>