assert that TMP always picks a non-empty candidate; clean up code a bit; consistently use super.size(info, writer) not info.sizeInBytes

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/branch_5x@1630050 13f79535-47bb-0310-9956-ffa450edef68
avoid pathological merging

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/branch_5x@1630064 13f79535-47bb-0310-9956-ffa450edef68