create_bundle(): duplicate file descriptor to avoid closing it twice

create_bundle(): duplicate file descriptor to avoid closing it twice

write_pack_data() passes bundle_fd to start_command() to be used as
the stdout of pack-objects. But start_command() closes its stdout if
it is > 1. This is a problem if bundle_fd is the fd of a lock_file,
because commit_lock_file() will also try to close the fd.

So the old code suppressed commit_lock_file()'s usual behavior of
closing the file descriptor by setting the lock_file object's fd field
to -1.

But this is not really kosher. Code here shouldn't be mutating fields
within the lock_file object.

Instead, duplicate the file descriptor before passing it to
write_pack_data(). Then that function can close its copy without
closing the copy held in the lock_file object.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>