checkout: retire --to option

checkout: retire --to option

Now that "git worktree add" has achieved user-facing feature-parity with
"git checkout --to", retire the latter.

Move the actual linked worktree creation functionality,
prepare_linked_checkout() and its helpers, verbatim from checkout.c to
worktree.c.

This effectively reverts changes to checkout.c by 529fef2 (checkout:
support checking out into a new working directory, 2014-11-30) with the
exception of merge_working_tree() and switch_branches() which still
require specialized knowledge that a the checkout is occurring in a
newly-created linked worktree (signaled to them by the private
GIT_CHECKOUT_NEW_WORKTREE environment variable).

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