SOLR-6342: build.xml clones lucene/CHANGES.txt as LUCENE_CHANGES.txt in solr releases; changes2html hotlinks LUCENE_CHANGES.txt when found (merge r1618281)

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/branch_4x@1618284 13f79535-47bb-0310-9956-ffa450edef68
SOLR-5868: fixing botched changes.txt placement/merge from 4.9 (merge r1618285)

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/branch_4x@1618286 13f79535-47bb-0310-9956-ffa450edef68