Fix alias confliction when joining same table on has many through with left_joins

This regression was caused by #30995 due to `Hash#fetch` won't invoke
default proc. Just revert the change since #30995 is completely fixed by
e9c1653.

Fixes #33048.

activerecord/lib/active_record/associations/alias_tracker.rb

@@ -33,7 +33,7 @@ module ActiveRecord
           elsif join.is_a?(Arel::Nodes::Join)
             join.left.name == name ? 1 : 0
           elsif join.is_a?(Hash)
-            join.fetch(name, 0)
+            join[name]
           else
             raise ArgumentError, "joins list should be initialized by list of Arel::Nodes::Join"
           end

activerecord/test/cases/associations/has_many_through_associations_test.rb

@@ -1289,6 +1289,10 @@ class HasManyThroughAssociationsTest < ActiveRecord::TestCase
     assert_equal authors(:david), Author.joins(:comments_for_first_author).take
   end
 
+  def test_has_many_through_with_left_joined_same_table_with_through_table
+    assert_equal [comments(:eager_other_comment1)], authors(:mary).comments.left_joins(:post)
+  end
+
   def test_has_many_through_with_unscope_should_affect_to_through_scope
     assert_equal [comments(:eager_other_comment1)], authors(:mary).unordered_comments
   end