avoid double close of descriptors handed to run_command

avoid double close of descriptors handed to run_command

When a file descriptor is given to run_command via the
"in", "out", or "err" parameters, run_command takes
ownership. The descriptor will be closed in the parent
process whether the process is spawned successfully or not,
and closing it again is wrong.

In practice this has not caused problems, because we usually
close() right after start_command returns, meaning no other
code has opened a descriptor in the meantime. So we just get
EBADF and ignore it (rather than accidentally closing
somebody else's descriptor!).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>