How can I tell Emacs my git branch has changed?

At the bottom of my Emacs 23 editor, I notice that Emacs is aware that I am working in a directory that is under version control, what that version control system is, and what branch I am currently on. Pretty cool! But say I am on the master branch, and from the command line I do a git commit, followed by a git checkout <branch>. Emacs still shows me being on the master branch. How do I refresh Emacs so that it reflects the branch I am currently on without closing down all my buffers and restarting it?