History: increment @original_lines

Fix broken `Pry::History#session_line_count`. Add a test.

lib/pry/history.rb

@@ -28,6 +28,7 @@ class Pry
       @loader.call do |line|
         @pusher.call(line.chomp)
         @history << line.chomp
+        @original_lines += 1
       end
     end
 

spec/history_spec.rb

@@ -17,6 +17,7 @@ describe Pry do
   after do
     Pry.history.clear
     Pry.history.restore_default_behavior
+    Pry.history.instance_variable_set(:@original_lines, 0)
   end
 
   describe '#push' do
@@ -34,6 +35,14 @@ describe Pry do
     end
   end
 
+  describe "#session_line_count" do
+    it "returns the number of lines in history from just this session" do
+      Pry.history << 'you?'
+      Pry.history << 'you are so precious'
+      Pry.history.session_line_count.should == 2
+    end
+  end
+
   describe ".load_history" do
     it "should read the contents of the file" do
       Pry.history.to_a[-2..-1].should == %w(2 3)