Highlighter mergeContiguousFragments shouldn't merge 0-score fragments [LUCENE-6471]

Highlighter.mergeContiguousFragments merges adjacent fragments it is given. But it is given a list of fragments that do not necessarily have embedded highlights (e.g. have a score of 0), and so it could grow a fragment needlessly. I never figured out why this old highlighter keeps around such fragments instead of eagerly tossing them when the fragment completes, which is what I think it should do. That would address this problem and might make things faster. I'm not sure if any highlighter user wants the non-scoring fragments though.

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Migrated from [LUCENE-6471](https://issues.apache.org/jira/browse/LUCENE-6471) by David Smiley (@dsmiley), 1 vote, updated Jul 11 2017

Locate Highlighter.mergeContiguousFragments and inspect its callers and existing tests. Reproduce the behavior with adjacent zero-score fragments, then determine from the surrounding highlighter behavior whether they should be discarded or preserved. Done means the intended handling is covered by a regression test without unnecessarily merging zero-score fragments.

Written by the indexing model from the issue text.