disjunction searcher avoids second ID.Equals() comparison
disjunction searcher avoids second ID.Equals() comparison

Optimization for DisjunctionSearcher, where an extra matchingIdxs
helps track the currs that were matching.  This avoids the previous
code's second loop through the currs slice.

steveyen/bleve,steveyen/bleve,blevesearch/bleve,slavikm/bleve,slavikm/bleve,blevesearch/bleve