Use 'checkout' instead of 'track' to follow upstream branches.

Tracking a branch created by git-svn [no longer works][track-broken] as of git 1.8.3.2, since there's no valid upstream remote.

[track-broken]: http://git.661346.n2.nabble.com/git-svn-Use-prefix-by-default-td7594288.html#a7597159