The command_close_bidi_pipe() function will insist on closing both
input and output pipes returned by command_bidi_pipe(). With this
change it is possible to close one of the pipes in advance and pass
undef as an argument.
Signed-off-by: Michal Nazarewicz <mina86@mina86.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>