Fix path for searching npm-installed binary when in worktree

[Envek]

Fixes #192

The problem arises when:

1. using worktrees
2. lefthook is installed via NPM and isn't available in $PATH
3. lefthook version in main worktree's package.json is different from version in the current worktree

Then previously used code always return path to main worktree and founds old Lefthook binary there.

Current code will return path to the root of currently used worktree.

In most cases plain pwd -P would be enough but docs for git hooks states that:

Before Git invokes a hook, it changes its working directory to either $GIT_DIR in a bare repository or the root of the working tree in a non-bare repository. An exception are hooks triggered during a push (pre-receive, update, post-receive, post-update, push-to-checkout) which are always executed in $GIT_DIR.

While Lefthook is always used in non-bare repositories, there is still chance that someone would like to use mentioned hooks that triggered during push, so use git rev-parse --show-toplevel helper command instead. Just in case.

Relevant docs:

• git rev-parse --show-toplevel
• git hooks