push: further clean up fields of "struct ref"

push: further clean up fields of "struct ref"

The "nonfastforward" and "update" fields are only used while
deciding what value to assign to the "status" locally in a single
function.  Remove them from the "struct ref".

The "requires_force" field is not used to decide if the proposed
update requires a --force option to succeed, or to record such a
decision made elsewhere.  It is used by status reporting code that
the particular update was "forced".  Rename it to "forced_update",
and move the code to assign to it around to further clarify how it
is used and what it is used for.

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