Hack for readline test. (TODO: fix ext-readline's test)

Author: tompng

lib/reline.rb

@@ -341,6 +341,7 @@ def readline(prompt = '', add_hist = false)
         io_gate.set_default_key_bindings(config)
       end
 
+      line_editor.print_nomultiline_prompt(prompt)
       line_editor.rerender
 
       begin

lib/reline/line_editor.rb

@@ -414,6 +414,14 @@ def render_full_content
     @output.puts wrapped_lines.flatten.map { |l| "#{l}\r\n" }.join
   end
 
+  def print_nomultiline_prompt(prompt)
+    return unless prompt && !@is_multiline
+
+    # Readline's test `TestRelineAsReadline#test_readline` requires first output to be prompt, not cursor reset escape sequence.
+    @rendered_screen_cache = [[[0, Reline::Unicode.calculate_width(prompt, true), prompt]]]
+    @output.write prompt
+  end
+
   def render_differential
     unless @dialog_initialzed
       update_dialogs