Add support for git-worktree

[f440]

lefthook can't recognize git-worktree directory.

mkdir repo
cd repo
git init

lefthook install
git add lefthook.yml && git commit -m "Initial commit"

git worktree add -b worktree-test-branch ../repo2
cd ../repo2
touch test.txt && git add test.txt && git commit -m "Update" test.txt # error!

This PR gives git-worktree support to lefthook.

[Arkweid]

Awesome @f440
Could you check the tests?

[f440]

Oh, I'm sorry about CI broken...