Revert "Optimize show_source command further"

This reverts commit 27dd286.

This is to fix the test I added.

(I separated commits to test a new behavior of ruby-commit-hook)

lib/irb/cmd/show_source.rb

@@ -61,15 +61,12 @@ 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[lnum]
+          code = lines[0..lnum].join
           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