Some run-command callers may wish to just discard any data that
is sent to stdout from the child. This is a lot like our existing
no_stdin support, we just open /dev/null and duplicate the descriptor
into position.
Signed-off-by: Shawn O. Pearce <spearce@spearce.org> Signed-off-by: Junio C Hamano <junkio@cox.net>