don't import errors if attempting to merge with self

Prior to this change, attempting to merge an `ActiveModel::Error` instance
with itself would result in an endless loop where `ActiveModel::NestedError`s
would continue to be imported on the instance until interrupted. Though the
merging of identical objects is less likely to happen in practice, this method
should still be able to handle such a case gracefully. This change ensures
that instances attempting to merge on themselves return early rather than
hanging indefinitely.

Addresses https://github.com/rails/rails/issues/43737

activemodel/lib/active_model/errors.rb

@@ -140,6 +140,8 @@ module ActiveModel
     #
     #   person.errors.merge!(other)
     def merge!(other)
+      return errors if equal?(other)
+
       other.errors.each { |error|
         import(error)
       }

activemodel/test/cases/errors_test.rb

@@ -657,6 +657,16 @@ class ErrorsTest < ActiveModel::TestCase
     assert(person.errors.added?(:name, :blank))
   end
 
+  test "merge does not import errors when merging with self" do
+    errors = ActiveModel::Errors.new(Person.new)
+    errors.add(:name, :invalid)
+    errors_before_merge = errors.dup
+
+    errors.merge!(errors)
+
+    assert(errors.errors.eql?(errors_before_merge.errors))
+  end
+
   test "errors are marshalable" do
     errors = ActiveModel::Errors.new(Person.new)
     errors.add(:name, :invalid)