Load history when starting a direct debug session

When starting a debug session directly with RUBY_DEBUG_IRB_CONSOLE=1 and
`require 'debug'; debugger`, IRB's history wasn't loaded. This commit ensures
history is loaded in this case by calling `load_history` when configuring IRB
for the debugger.

Fixes ruby#975

lib/irb/debug.rb

@@ -81,6 +81,7 @@ def configure_irb_for_debugger(irb)
         IRB.instance_variable_set(:@debugger_irb, irb)
         irb.context.with_debugger = true
         irb.context.irb_name += ":rdbg"
+        irb.context.io.load_history
       end
 
       module SkipPathHelperForIRB

test/irb/test_history.rb

@@ -488,6 +488,28 @@ def foo
       HISTORY
     end
 
+    def test_direct_debug_session_loads_history
+      @envs['RUBY_DEBUG_IRB_CONSOLE'] = "1"
+      write_history <<~HISTORY
+        old_history_1
+        old_history_2
+        old_history_3
+      HISTORY
+
+      write_ruby <<~'RUBY'
+        require 'debug'
+        debugger
+        binding.irb # needed to satisfy run_ruby_file
+      RUBY
+
+      output = run_ruby_file do
+        type "history"
+        type "exit!"
+      end
+
+      assert_include(output, "old_history_3")
+    end
+
     private
 
     def write_history(history)