use result of system directly instead of calling "svn help" with redirects.

because it may cause permission problem on cygwin.
see https://twitter.com/higaki/status/322542075927212034

git-svn-id: http://jp.rubyist.net/svn/rurema/bitclust/trunk@5770 018432c1-2f1c-0410-ba7c-d424024924b7