checkout: relocate --to's "no branch specified" check
The plan is to relocate "git checkout --to" functionality to "git
worktree add", however, this check expects a 'struct branch_info' which
git-worktree won't have at hand. It will, however, have access to its
own command-line from which it can pick up the branch name. Therefore,
as a preparatory step, rather than having prepare_linked_checkout()
perform this check, make it the caller's responsibility.
Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>