pull: give the advice for choosing rebase/merge much later

pull: give the advice for choosing rebase/merge much later

Eventually we want to be omit the advice when we can fast-forward
in which case there is no reason to require the user to choose
between rebase or merge.

In order to do so, we need to delay giving the advice up to the
point where we can check if we can fast-forward or not.

Additionally, config_get_rebase() was probably never its true home.

Signed-off-by: Felipe Contreras <redacted>
Signed-off-by: Junio C Hamano <redacted>