worktree: fix "add -B"

Current code does not update "symref" when -B is used. This string
contains the new HEAD. Because it's empty "git worktree add -B" fails at
symbolic-ref step.

Because branch creation is already done before calling add_worktree(),
-B is equivalent to -b from add_worktree() point of view. We do not need
the special case for -B.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
