[ruby/irb] Powerup show_source by enabling RubyVM.keep_script_lines

(https://github.com/ruby/irb/pull/862)

* Powerup show_source by enabling RubyVM.keep_script_lines

* Add file_content field to avoid reading file twice while show_source

* Change path passed to eval, don't change irb_path.

* Encapsulate source coloring logic and binary file check insode class Source

* Add edit command testcase when irb_path does not exist

* Memoize irb_path existence to reduce file existence check calculating eval_path

239683a937

lib/irb/cmd/show_source.rb

@@ -45,15 +45,18 @@ module IRB
       private
 
       def show_source(source)
-        file_content = IRB::Color.colorize_code(File.read(source.file))
-        code = file_content.lines[(source.first_line - 1)...source.last_line].join
-        content = <<~CONTENT
+        if source.binary_file?
+          content = "\n#{bold('Defined in binary file')}: #{source.file}\n\n"
+        else
+          code = source.colorized_content || 'Source not available'
+          content = <<~CONTENT
 
-          #{bold("From")}: #{source.file}:#{source.first_line}
+            #{bold("From")}: #{source.file}:#{source.line}
 
-          #{code}
-        CONTENT
+            #{code.chomp}
 
+          CONTENT
+        end
         Pager.page_content(content)
       end