Reset history counter even when @loaded_history_lines is not defined

The issue (https://github.com/ruby/debug/issues/1064) is caused by a
combination of factors:

1. When user starts an IRB session without a history file, the
   `@loaded_history_lines` ivar is not defined.
2. If the user then starts the `irb:rdbg` session, the history counter
   is not set, because the `@loaded_history_lines` is not defined.
3. Because `irb:rdbg` saves the history before passing Ruby expression
   to the debugger, it saves the history with duplicated lines. The number
   grows in exponential order.
4. When the user exits the `irb:rdbg` session, the history file could be
   bloated with duplicated lines.

This commit fixes the issue by resetting the history counter even when
`@loaded_history_lines` is not defined.

lib/irb/history.rb

@@ -5,7 +5,7 @@ def support_history_saving?
     end
 
     def reset_history_counter
-      @loaded_history_lines = self.class::HISTORY.size if defined? @loaded_history_lines
+      @loaded_history_lines = self.class::HISTORY.size
     end
 
     def load_history

test/irb/test_history.rb

@@ -251,10 +251,6 @@ def with_temp_stdio
 
   class IRBHistoryIntegrationTest < IntegrationTestCase
     def test_history_saving_with_debug
-      if ruby_core?
-        omit "This test works only under ruby/irb"
-      end
-
       write_history ""
 
       write_ruby <<~'RUBY'
@@ -293,6 +289,51 @@ def foo
       HISTORY
     end
 
+    def test_history_saving_with_debug_without_prior_history
+      tmpdir = Dir.mktmpdir("test_irb_history_")
+      # Intentionally not creating the file so we test the reset counter logic
+      history_file = File.join(tmpdir, "irb_history")
+
+      write_rc <<~RUBY
+        IRB.conf[:HISTORY_FILE] = "#{history_file}"
+      RUBY
+
+      write_ruby <<~'RUBY'
+        def foo
+        end
+
+        binding.irb
+
+        foo
+      RUBY
+
+      output = run_ruby_file do
+        type "'irb session'"
+        type "next"
+        type "'irb:debug session'"
+        type "step"
+        type "irb_info"
+        type "puts Reline::HISTORY.to_a.to_s"
+        type "q!"
+      end
+
+      assert_include(output, "InputMethod: RelineInputMethod")
+      # check that in-memory history is preserved across sessions
+      assert_include output, %q(
+        ["'irb session'", "next", "'irb:debug session'", "step", "irb_info", "puts Reline::HISTORY.to_a.to_s"]
+      ).strip
+
+      assert_equal <<~HISTORY, File.read(history_file)
+        'irb session'
+        next
+        'irb:debug session'
+        step
+        irb_info
+        puts Reline::HISTORY.to_a.to_s
+        q!
+      HISTORY
+    end
+
     def test_history_saving_with_nested_sessions
       write_history ""