A lot. (Coauthor here.)

Not every sentence was rewritten, but 4 years is a long time. There was a lot of content that was either inaccurate or out of date. We added content about two-way bridges and migration to other VCSes, graphical clients, shell integration, and lots more. There are also new chapters on GitHub and embedded Git (Libgit2 and JGit).

Chacon sketches the differences in the Preface. As I understand the Preface (I haven't read the new book nor compared the respective TOCs), the second edition aims to give deeper treatments of these three topics:

1. HTTP as the simplest protocol for Git network transactions

2. GitHub and the overall Git community

3. Git on MS Windows