[Q] Changing the current branch (HEAD) in a bare repository - how?

 Using git 1.5.2.5 (Kubuntu 7.10) and a bare repository,
 I started with (the commands and output below are typed
 in by hand, mostly from memory):

    $ git branch -a
    * master
      foo
    $

 and then changed the branch names:

    $ git branch -m master old-master
    $ git branch -m foo master

 and got this:

    $ git branch -a
      master
    * old-master
    $

 Now, how do I change the current branch to the (new)
 `master'?  A `checkout' fails:

    $ git checkout master
    fatal: /usr/bin/git-checkout cannot be used without a working tree.
    $

 I (ultimately) hand-edited `HEAD' (apparently successfully),
 but am wondering what I should have done or may have missed?

 I didn't spot anything in TFM, albeit I could have easily overlooked
 or misunderstood something.

cheers!
	-blf-