properly handle extra identifiers in cvs tags for releases

as described in http://drupal.org/node/90436#comment-169221 (point #3), we're not properly handling CVS tags that define extra identifiers, e.g. "DRUPAL-5--1-0-BETA-1". the solution is easy: in cvs_get_version_from_tag(), when we're using explode() on the part of the tag name after the '--', we just want an argument so that we explode into at most 3 things: (major, patch, extra). then, if extra contains an '-' chars, they're all still included in extra (and our handy _cvs_get_extra() method will properly format it for us). we already do this trick for core, we just need to do it for contrib, too.