Support for git worktrees

[er2]

https://github.com/randomcoder/sbt-git-hooks/blob/f824621547f987a3a383f37d414ba1434ddcbc99/src/main/scala/uk/co/randomcoding/sbt/GitHooks.scala#L42

This will fail if the repo is a worktree, in which case .git is a file and hooks are stored in a bare repository. To find that bare repository you can parse the output of git worktree list --porcelain