Aggregations: Geohash grid should count documents only once

[jpountz]

If two geo-hashes of the same document fall into the same bucket, then the document count for this bucket will be incremented by 2 instead of 1. Here is a reproduction:

DELETE test

PUT test
{
  "mappings": {
    "test": {
      "properties": {
        "points": {
          "type": "geo_point"
        }
      }
    }
  }
}

PUT test/test/1
{
  "points": [
    "1,2",
    "2,3"
  ]
}

GET /test/_search?search_type=count
{
  "aggs": {
    "grid": {
      "geohash_grid": {
        "field": "points",
        "precision": 2
      }
    }
  }
}

which returns:

{
   "took": 3,
   "timed_out": false,
   "_shards": {
      "total": 5,
      "successful": 5,
      "failed": 0
   },
   "hits": {
      "total": 1,
      "max_score": 0,
      "hits": []
   },
   "aggregations": {
      "grid": {
         "buckets": [
            {
               "key": "s0",
               "doc_count": 2
            }
         ]
      }
   }
}

[ejain]

btw the geohash grid aggregation fails in 1.4.0 if documents with more than one value are present, see #8507

[jpountz]

Indeed, I found this issue while working on a fix for that issue! :) #8513