Now that the code has been simplified and it's clear what it's
actually doing, update the documentation to reflect that.
Namely; the simple mode only barfs when working on a centralized
workflow, and there's no configured upstream branch with the same name.
Cc: Elijah Newren <newren@gmail.com>
Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
90cfb2666b