Add --set-upstream option to branch that works like --track, except that
when branch exists already, its upstream info is changed without changing
the ref value.

Based-on-patch-from: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: NMatthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: NIlari Liusvaara <ilari.liusvaara@elisanet.fi>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>