HBASE-26398 CellCounter fails for large tables filling up local disk (apache#3798)

Signed-off-by: Wellington Chevreuil <wchevreuil@apache.org>
(cherry picked from commit 60254bc)

Author: stoty

hbase-mapreduce/src/main/java/org/apache/hadoop/hbase/mapreduce/CellCounter.java

@@ -215,14 +215,14 @@ public static Job createSubmittableJob(Configuration conf, String[] args)
     Scan scan = getConfiguredScanForJob(conf, args);
     TableMapReduceUtil.initTableMapperJob(tableName, scan,
         CellCounterMapper.class, ImmutableBytesWritable.class, Result.class, job);
-    job.setNumReduceTasks(1);
     job.setMapOutputKeyClass(Text.class);
     job.setMapOutputValueClass(IntWritable.class);
     job.setOutputFormatClass(TextOutputFormat.class);
     job.setOutputKeyClass(Text.class);
     job.setOutputValueClass(IntWritable.class);
     FileOutputFormat.setOutputPath(job, outputDir);
     job.setReducerClass(IntSumReducer.class);
+    job.setCombinerClass(IntSumReducer.class);
     return job;
   }