Wasn't the commit button added recently (r29433) on request [1] from someone was using this dialog to find changed files?
If you are comparing the button placements between TortoiseGit and TortoiseSvn, they share ancestry but not the same codebase (although I know Stefan has been contributing to TortoiseGit as well). On the screenshot in [1] the order between the Commit and Refresh buttons are the same in both TSVN and TGIT, but TSVN has the "Check repository" button which doesn't exist in TGIT (since the working copy is also the repository in Git). I would argue that the current button placement makes the most sense, not to kill the muscle memory of anyone using Refresh and Check repository.