restore: reject invalid combinations with --staged

restore: reject invalid combinations with --staged

git-checkout rejects plenty of invalid option combinations. Since
git-checkout is equivalent of either

    git restore --source --staged --worktree

or

    git restore --worktree

that still leaves the new mode 'git restore --index' unprotected. Reject
some more invalid option combinations.

The other new mode 'restore --source --worktree' does not need anything
else.

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