checkout: require worktree unconditionally

checkout: require worktree unconditionally

In order to allow linked worktree creation via "git checkout --to" from
a bare repository, 3473ad0 (checkout: don't require a work tree when
checking out into a new one, 2014-11-30) dropped git-checkout's
unconditional NEED_WORK_TREE requirement and instead performed worktree
setup conditionally based upon presence or absence of the --to option.
Now that --to has been retired and git-checkout is no longer responsible
for linked worktree creation, the NEED_WORK_TREE requirement can be
re-instated.

This effectively reverts 3473ad0, except for the tests it added which
now check bare repository behavior of "git worktree add" instead.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>