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>

builtin/checkout.c

@@ -1218,8 +1218,6 @@ int cmd_checkout(int argc, const char **argv, const char *prefix)
 
 	opts.new_worktree_mode = getenv("GIT_CHECKOUT_NEW_WORKTREE") != NULL;
 
-	setup_work_tree();
-
 	if (conflict_style) {
 		opts.merge = 1; /* implied */
 		git_xmerge_config("merge.conflictstyle", conflict_style, NULL);

git.c

@@ -383,7 +383,7 @@ static struct cmd_struct commands[] = {
 	{ "check-ignore", cmd_check_ignore, RUN_SETUP | NEED_WORK_TREE },
 	{ "check-mailmap", cmd_check_mailmap, RUN_SETUP },
 	{ "check-ref-format", cmd_check_ref_format },
-	{ "checkout", cmd_checkout, RUN_SETUP },
+	{ "checkout", cmd_checkout, RUN_SETUP | NEED_WORK_TREE },
 	{ "checkout-index", cmd_checkout_index,
 		RUN_SETUP | NEED_WORK_TREE},
 	{ "cherry", cmd_cherry, RUN_SETUP },