Simplify closing two fds at once in run-command.c

Simplify closing two fds at once in run-command.c

I started hacking on a change to add stdout redirection support to
the run_command family, but found I was using a lot of close calls
on two pipes in an array (such as for pipe).  So I'm doing a tiny
bit of refactoring first to make the next set of changes clearer.

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