Fixed #19102 -- Fixed fast-path delete for modified SELECT clause cases

There was a bug introduced in #18676 which caused fast-path deletes
implemented as "DELETE WHERE pk IN <subquery>" to fail if the SELECT
clause contained additional stuff (for example extra() and annotate()).

Thanks to Trac alias pressureman for spotting this regression.

django/db/models/sql/query.py

@@ -431,13 +431,9 @@ class Query(object):
 
     def has_results(self, using):
         q = self.clone()
+        q.clear_select_clause()
         q.add_extra({'a': 1}, None, None, None, None, None)
-        q.select = []
-        q.select_fields = []
-        q.default_cols = False
-        q.select_related = False
-        q.set_extra_mask(('a',))
-        q.set_aggregate_mask(())
+        q.set_extra_mask(['a'])
         q.clear_ordering(True)
         q.set_limits(high=1)
         compiler = q.get_compiler(using=using)
@@ -1626,6 +1622,17 @@ class Query(object):
         """
         return not self.low_mark and self.high_mark is None
 
+    def clear_select_clause(self):
+        """
+        Removes all fields from SELECT clause.
+        """
+        self.select = []
+        self.select_fields = []
+        self.default_cols = False
+        self.select_related = False
+        self.set_extra_mask(())
+        self.set_aggregate_mask(())
+
     def clear_select_fields(self):
         """
         Clears the list of fields to select (but not extra_select columns).