restore: fault --staged --worktree with merge opts

restore: fault --staged --worktree with merge opts

The 'restore' command already rejects the --merge, --conflict, --ours
and --theirs options when combined with --staged, but accepts them when
--worktree is added as well.

Unfortunately that doesn't appear to do anything useful. The --ours and
--theirs options seem to be ignored when both --staged and --worktree
are given, whereas with --merge or --conflict, the command has the same
effect as if the --staged option wasn't present.

So reject those options with '--staged --worktree' as well, using
opts->accept_ref to distinguish restore from checkout.

Add test for both '--staged' and '--staged --worktree'.

Signed-off-by: Andy Koppe <andy.koppe@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>