StringBuffer => StringBuilder where possible

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/exec/trunk@1557379 13f79535-47bb-0310-9956-ffa450edef68
Unused

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/exec/trunk@1557380 13f79535-47bb-0310-9956-ffa450edef68