[ruby/irb] Optimize show_source command further

ruby/irb#249 actually slowed down how `code` is
concatenated. The original way of creating `code` is faster.

[before]
    user     system      total        real
2.420137   0.005364   2.425501 (  2.426264)

[after]
    user     system      total        real
1.000221   0.007454   1.007675 (  1.008295)

Theoretically, this implementation might skip lines that don't appear in
Ripper tokens, but this assumes such lines don't impact whether the code
passes compilation or not. At least normal blank lines seem to have an
`on_ignored_nl` token anyway though.

ruby/irb@27dd2867cd

lib/irb/cmd/show_source.rb

@@ -61,12 +61,15 @@ def find_end(file, first_line)
         lex = RubyLex.new
         lines = File.read(file).lines[(first_line - 1)..-1]
         tokens = RubyLex.ripper_lex_without_warning(lines.join)
+
+        code = +""
         prev_tokens = []
 
         # chunk with line number
         tokens.chunk { |tok| tok[0][0] }.each do |lnum, chunk|
-          code = lines[0..lnum].join
+          code << lines[lnum]
           prev_tokens.concat chunk
+
           continue = lex.process_continue(prev_tokens)
           code_block_open = lex.check_code_block(code, prev_tokens)
           if !continue && !code_block_open