setup_git_directory(): fix move to worktree toplevel directory

setup_git_directory(): fix move to worktree toplevel directory

When setup_git_directory() returns successfully, it is supposed to move
current working directory to worktree toplevel directory.

However, the code recomputing prefix inside setup_git_directory() has
to move cwd back to original working directory, in order to get new
prefix.  After that, it should move cwd back to worktree toplevel
directory as expected.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>