Fix history clear bag(No re-init @original_lines)

lib/pry/history.rb

@@ -57,6 +57,7 @@ class Pry
     # history file.
     def clear
       @clearer.call
+      @original_lines = 0
       @history = []
     end
 

spec/history_spec.rb

@@ -53,6 +53,7 @@ describe Pry do
       Pry.history.to_a.size.should > 0
       Pry.history.clear
       Pry.history.to_a.size.should == 0
+      Pry.history.original_lines.should == 0
     end
 
     it "doesn't affect the contents of the history file" do