setup: return correct prefix if worktree is '/'

setup: return correct prefix if worktree is '/'

The same old problem reappears after setup code is reworked.  We tend
to assume there is at least one path component in a path and forget
that path can be simply '/'.

Reported-by: Matthijs Kooijman <matthijs@stdin.nl>
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>