Suppress DISTINCT clause outside aggregate function

`DISTINCT` clause is applied inside aggregate function by
`operation_over_aggregate_column` if needed. Unneeded outside aggregate
function.

```ruby
  # Before
  author.unique_categorized_posts.count
  # => SELECT DISTINCT COUNT(DISTINCT "posts"."id") FROM "posts" INNER JOIN "categorizations" ON "posts"."id" = "categorizations"."post_id" WHERE "categorizations"."author_id" = ?  [["author_id", 2]]

  # After
  author.unique_categorized_posts.count
  # => SELECT COUNT(DISTINCT "posts"."id") FROM "posts" INNER JOIN "categorizations" ON "posts"."id" = "categorizations"."post_id" WHERE "categorizations"."author_id" = ?  [["author_id", 2]]
```

Closes rails#27615

Author: kamipo

activerecord/lib/active_record/relation/calculations.rb

@@ -232,7 +232,7 @@ def execute_simple_calculation(operation, column_name, distinct) #:nodoc:
           query_builder = build_count_subquery(spawn, column_name, distinct)
         else
           # PostgreSQL doesn't like ORDER BY when there are no GROUP BY
-          relation = unscope(:order)
+          relation = unscope(:order).distinct!(false)
 
           column = aggregate_column(column_name)
 
@@ -292,7 +292,7 @@ def execute_grouped_calculation(operation, column_name, distinct) #:nodoc:
           end
         }
 
-        relation = except(:group)
+        relation = except(:group).distinct!(false)
         relation.group_values  = group_fields
         relation.select_values = select_values
 

activerecord/test/cases/calculations_test.rb

@@ -227,6 +227,17 @@ def test_count_on_invalid_columns_raises
     assert_match "credit_limit, firm_name", e.message
   end
 
+  def test_apply_distinct_in_count
+    queries = assert_sql do
+      Account.distinct.count
+      Account.group(:firm_id).distinct.count
+    end
+
+    queries.each do |query|
+      assert_match %r{\ASELECT(?! DISTINCT) COUNT\(DISTINCT\b}, query
+    end
+  end
+
   def test_should_group_by_summed_field_having_condition
     c = Account.group(:firm_id).having("sum(credit_limit) > 50").sum(:credit_limit)
     assert_nil        c[1]