LUCENE-5904: don't parse segments.gen as a segments file and overinflate

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/lucene5904@1620777 13f79535-47bb-0310-9956-ffa450edef68
LUCENE-5904: test segments inflation

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/lucene5904@1620778 13f79535-47bb-0310-9956-ffa450edef68