If, for some reason, a hook looked at stdin, it could get surprising
results.
In the first patch, the hook would hang waiting for input. That was a
bug. In the second patch, the hook's standard input has been closed
by git-send-pack (which is the standard thing for the parent to do in
a pipeline), thus if a hook looks at stdin it will get EOF (the same
as if connected to /dev/null).
Paul