branch.c: simplify advice-and-die sequence

In the dwim_branch_start(), when we cannot find an appropriate
upstream, we will die with the same message anyway, whether we
issue an advice message.

Flip the code around a bit and simplify the flow using
advise_if_enabled() function.

Helped-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Glen Choo <chooglen@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>