checkout: teach check_linked_checkout() about symbolic link HEAD
check_linked_checkout() only understands symref-style HEAD (i.e. "ref:
refs/heads/master"), however, HEAD may also be a an actual symbolic link
(on platforms which support it). To accurately detect if a branch is
checked out elsewhere, it needs to handle symbolic link HEAD, as well.
Signed-off-by: Eric Sunshine <sunshine@sunshineco.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>