Fix sizes before gc and add change of branch

* We need 'git checkout master' because we've left on 'test' branch
  in previous section
* I was surprised that before gc we had only 22K objects, because
  we have two copies of 22K files. So I checked it and it appears
  that because of compression each of them takes only 7K place
  on disk. Also before compression all objects take 15K if summing
  them up (not 22K).

Author: hedrok

book/10-git-internals/sections/packfiles.asc

@@ -26,6 +26,7 @@ To demonstrate, we'll add the `repo.rb` file from the Grit library – this is a
 [source,console]
 ----
 $ curl https://raw.githubusercontent.com/mojombo/grit/master/lib/grit/repo.rb > repo.rb
+$ git checkout master
 $ git add repo.rb
 $ git commit -m 'added repo.rb'
 [master 484a592] added repo.rb
@@ -81,7 +82,7 @@ $ git cat-file -s b042a60ef7dff760008df33cee372b945b6e884e
 22054
 ----
 
-You have two nearly identical 22K objects on your disk.
+You have two nearly identical 22K objects on your disk (each compressed to approximately 7K).
 Wouldn't it be nice if Git could store one of them in full but then the second object only as the delta between it and the first?
 
 It turns out that it can.
@@ -118,8 +119,8 @@ Because you never added them to any commits, they're considered dangling and are
 The other files are your new packfile and an index.
 The packfile is a single file containing the contents of all the objects that were removed from your filesystem.
 The index is a file that contains offsets into that packfile so you can quickly seek to a specific object.
-What is cool is that although the objects on disk before you ran the `gc` were collectively about 22K in size, the new packfile is only 7K.
-You've cut your disk usage by ⅔ by packing your objects.
+What is cool is that although the objects on disk before you ran the `gc` were collectively about 15K in size, the new packfile is only 7K.
+You've cut your disk usage by ½ by packing your objects.
 
 How does Git do this?
 When Git packs objects, it looks for files that are named and sized similarly, and stores just the deltas from one version of the file to the next.