push: further simplify the logic to assign rejection reason

push: further simplify the logic to assign rejection reason

First compute the reason why this push would fail if done without
"--force", and then fail it by assigning that reason when the push
was not forced (or if there is no reason to require force, allow it
to succeed).

Record the fact that the push was forced in the forced_update field
only when the push would have failed without the option.

The code becomes shorter, less repetitive and easier to read this
way, especially given that the set of rejection reasons will be
extended in a later patch.

Signed-off-by: Junio C Hamano <gitster@pobox.com>