[ruby/irb] Fix history file saving with concurrent irb sessions when history file doesn't exist

If history file didn't exist when irb was started, @loaded_history_mtime
would be nil.  However, if the history file didn't exist before, but it
exists when saving history, that means the history file was modified,
and we should handle it the same way as we handle the other case where
the history file was modified.

Fixes #388

https://github.com/ruby/irb/commit/8d277aafcb

lib/irb/ext/save-history.rb

@@ -107,9 +107,9 @@ module IRB
           raise
         end
 
-        if File.exist?(history_file) && @loaded_history_mtime &&
+        if File.exist?(history_file) &&
            File.mtime(history_file) != @loaded_history_mtime
-          history = history[@loaded_history_lines..-1]
+          history = history[@loaded_history_lines..-1] if @loaded_history_lines
           append_history = true
         end
 

test/irb/test_history.rb

@@ -158,6 +158,28 @@ module TestIRB
       end
     end
 
+    def test_history_concurrent_use_not_present
+      backup_home = ENV["HOME"]
+      backup_xdg_config_home = ENV.delete("XDG_CONFIG_HOME")
+      IRB.conf[:SAVE_HISTORY] = 1
+      Dir.mktmpdir("test_irb_history_#{$$}") do |tmpdir|
+        ENV["HOME"] = tmpdir
+        io = TestInputMethod.new
+        io.class::HISTORY.clear
+        io.load_history
+        io.class::HISTORY.concat(%w"line1 line2")
+
+        history_file = IRB.rc_file("_history")
+        assert !File.file?(history_file)
+        File.write(history_file, "line0\n")
+        io.save_history
+        assert_equal(%w"line0 line1 line2", File.read(history_file).split)
+      end
+    ensure
+      ENV["HOME"] = backup_home
+      ENV["XDG_CONFIG_HOME"] = backup_xdg_config_home
+    end
+
     private
 
     def assert_history(expected_history, initial_irb_history, input)