worktree: add: suppress auto-vivication with --detach and no <branch>
Fix oversight where branch auto-vivication incorrectly kicks in when
--detach is specified and <branch> omitted. Instead, treat:
git worktree add --detach <path>
as shorthand for:
git worktree add --detach <path> HEAD
Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>