[1.8.x] Fixed #24835 -- Fixed QuerySet.exists() after an annotation with Count()

QuerySet.exists() incorrectly handled query.group_by = True
case (grouping by all select fields), causing GROUP BY
expressions to be wiped along with select fields.

Backport of 801a84ae32 from master

django/db/models/sql/query.py

@@ -472,6 +472,9 @@ class Query(object):
     def has_results(self, using):
         q = self.clone()
         if not q.distinct:
+            if q.group_by is True:
+                q.add_fields((f.attname for f in self.model._meta.concrete_fields), False)
+                q.set_group_by()
             q.clear_select_clause()
         q.clear_ordering(True)
         q.set_limits(high=1)