push: copy code to setup_push_simple()

push: copy code to setup_push_simple()

In order to avoid doing unnecessary things and simplify it in further
patches. In particular moving the additional name safety out of
setup_push_upstream() and into setup_push_simple() and thus making both
more straightforward.

The code is copied exactly as-is; no functional changes.

Reviewed-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>