Fix stashed count

The variable `git_dir` was not available in the context of the
function `git-info`. It was only defined in the function `_git-action`.

It is only used to determine if there is a stash and then set the stashed count: [git-info#L203](https://github.com/eugenk/prezto/blob/fix_stashed_count/modules/git/functions/git-info#L203)