worktree move: refuse to move worktrees with submodules

worktree move: refuse to move worktrees with submodules

Submodules contains .git files with relative paths. After a worktree
move, these files need to be updated or they may point to nowhere.

This is a bandage patch to make sure "worktree move" don't break
people's worktrees by accident. When .git file update code is in
place, this validate_no_submodules() could be removed.

Signed-off-by: Nguyễn Thái Ngọc Duy <redacted>
Signed-off-by: Junio C Hamano <redacted>