x/review/git-codereview: does not work with 'git worktree'

See https://github.com/blog/2042-git-2-5-including-multiple-worktrees-and-triangular-workflows for info about 'git worktree'.

At the least, the hook checker needs updating:

``` bash
$ git sync
git-codereview: checking hook: stat /Volumes/Data/src/go.ssa/.git/hooks/commit-msg: not a directory
```

There might be other stuff as well.
