Add support for worktrees to `x.py setup` git hook

[jyn514]

.git doesn't exist for worktrees. x.py setup should use git rev-parse --git-common-dir instead.

x.py encountered an error -- do you already have the git hook installed?
Not a directory (os error 20)

Originally posted by @jyn514 in #76356 (comment)

[jyn514]

Actually maybe this shouldn't run at all for worktrees since it applies globally to all worktrees and everything else x.py setup does currently is local to a worktree ...