Fix history file saving with concurrent irb sessions when history fil…

…e 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 ruby#388

lib/irb/ext/save-history.rb

@@ -107,7 +107,7 @@ def save_history
           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]
           append_history = true

test/irb/test_history.rb

@@ -158,6 +158,27 @@ def test_history_concurrent_use
       end
     end
 
+    def test_history_concurrent_use_not_present
+      omit "Skip Editline" if /EditLine/n.match(Readline::VERSION)
+      IRB.conf[:SAVE_HISTORY] = 1
+      assert_history(<<~EXPECTED_HISTORY, nil, <<~INPUT) do |history_file|
+        exit
+        5
+        exit
+      EXPECTED_HISTORY
+        5
+        exit
+      INPUT
+        assert_history(<<~EXPECTED_HISTORY2, nil, <<~INPUT2)
+        exit
+      EXPECTED_HISTORY2
+        5
+        exit
+      INPUT2
+        File.utime(File.atime(history_file), File.mtime(history_file) + 2, history_file)
+      end
+    end
+
     private
 
     def assert_history(expected_history, initial_irb_history, input)
@@ -168,8 +189,10 @@ def assert_history(expected_history, initial_irb_history, input)
       actual_history = nil
       Dir.mktmpdir("test_irb_history_#{$$}") do |tmpdir|
         ENV["HOME"] = tmpdir
-        open(IRB.rc_file("_history"), "w") do |f|
-          f.write(initial_irb_history)
+        if initial_irb_history
+          open(IRB.rc_file("_history"), "w") do |f|
+            f.write(initial_irb_history)
+          end
         end
 
         io = TestInputMethod.new