[ruby/reline] Fix completion with multiline

(https://github.com/ruby/reline/pull/513)

https://github.com/ruby/reline/commit/d76c482c5f

lib/reline/line_editor.rb

@@ -1371,8 +1371,8 @@ class Reline::LineEditor
         @completion_state = CompletionState::MENU
       end
       if not just_show_list and target < completed
-        @line = preposing + completed + completion_append_character.to_s + postposing
-        line_to_pointer = preposing + completed + completion_append_character.to_s
+        @line = (preposing + completed + completion_append_character.to_s + postposing).split("\n")[@line_index] || String.new(encoding: @encoding)
+        line_to_pointer = (preposing + completed + completion_append_character.to_s).split("\n").last || String.new(encoding: @encoding)
         @cursor_max = calculate_width(@line)
         @cursor = calculate_width(line_to_pointer)
         @byte_pointer = line_to_pointer.bytesize

test/reline/yamatanooroti/test_rendering.rb

@@ -839,6 +839,18 @@ begin
       EOC
     end
 
+    def test_multiline_completion
+      start_terminal(10, 50, %W{ruby -I#{@pwd}/lib #{@pwd}/test/reline/yamatanooroti/multiline_repl --complete}, startup_message: 'Multiline REPL.')
+      write("def hoge\n  St\n  St\C-p\t")
+      close
+      assert_screen(<<~'EOC')
+        Multiline REPL.
+        prompt> def hoge
+        prompt>   String
+        prompt>   St
+      EOC
+    end
+
     def test_completion_journey_2nd_line
       write_inputrc <<~LINES
         set editing-mode vi