Flush stdin from within chroot archive

This makes sure that we don't buffer in memory and that we also flush
stdin from diff as well as untar.

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>
idk, i only made it a function so that the name makes it clean what's it's doing instead of having the `io.Copy` in in the code.  Maybe if this is used other places it would be good to move but it maybe better to just keep it simple for now.