Fixes for JENKINS-13790 (svn:externals not read correctly) and JENKINS-16217 (post-commit hook fails on svn:externals)

Fixes JENKINS-13790: svn:externals not read correctly if the working copy is not v1.6 or the user has built their own version of svnkit.

Fixes JENKINS-16217: The svn post-commit hook fails to take into account the fact that a project may have a number of svn:externals.