Remove an extra branch from the bottleneck

RDoc::RubyLex#getc is the bottleneck in RDoc.  It determined which
ungetc buffer to use by branching upon `@here_header`.  We can remove
the branch by replacing `@here_header` flag with `@current_readed`,
which represents the current buffer directly,

lib/rdoc/ruby_lex.rb

@@ -101,10 +101,10 @@ def initialize(content, options)
     @exp_line_no = @line_no = 1
     @here_readed = []
     @readed = []
+    @current_readed = @readed
     @rests = []
     @seek = 0
 
-    @here_header = false
     @indent = 0
     @indent_stack = []
     @lex_state = :EXPR_BEG
@@ -160,7 +160,7 @@ def get_readed
     end
 
     readed = @readed.join("")
-    @readed = []
+    @readed.clear
     readed
   end
 
@@ -170,13 +170,9 @@ def getc
       @rests.push nil unless buf_input
     end
     c = @rests.shift
-    if @here_header
-      @here_readed.push c
-    else
-      @readed.push c
-    end
+    @current_readed.push c
     @seek += 1
-    if c == "\n"
+    if c == "\n".freeze
       @line_no += 1
       @char_no = 0
     else
@@ -282,7 +278,7 @@ def initialize_input
     @indent_stack = []
     @lex_state = :EXPR_BEG
     @space_seen = false
-    @here_header = false
+    @current_readed = @readed
 
     @continue = false
     prompt
@@ -461,8 +457,8 @@ def lex_init()
           @indent_stack.pop
         end
       end
-      @here_header = false
-      @here_readed = []
+      @current_readed = @readed
+      @here_readed.clear
       Token(TkNL)
     end
 
@@ -1020,7 +1016,7 @@ def identify_here_document
       doc = '"'
     end
 
-    @here_header = false
+    @current_readed = @readed
     while l = gets
       l = l.sub(/(:?\r)?\n\z/, "\n")
       if (indent ? l.strip : l.chomp) == quoted
@@ -1037,7 +1033,7 @@ def identify_here_document
       doc << '"'
     end
 
-    @here_header = true
+    @current_readed = @here_readed
     @here_readed.concat reserve
     while ch = reserve.pop
       ungetc ch