[FIXED JENKINS-6746] Updated to add setting of SVN_CURRENT_REVISION env

As per JENKINS-6746, the revision returned in SVN_REVISION is the revision from trunk but not the "current revision".  This change sets a new "SVN_CURRENT_REVISION" environmental variable to be the value of the "current revision" (basically, a maximum of any revisions involved in the repository section including any externals).  This allows, for example, the setting of the build number to be the current revision (so that it is unique if externals, for example, are updated but not trunk).

Note: Perhaps a couple of comment changes might be good to add...