push: fix "refs/tags/ hierarchy cannot be updated without --force"

When pushing to update a branch with a commit that is not a
descendant of the commit at the tip, a wrong message "already
exists" was given, instead of the correct "non-fast-forward", if we
do not have the object sitting in the destination repository at the
tip of the ref we are updating.

The primary cause of the bug is that the check in a new helper
function is_forwardable() assumed both old and new objects are
available and can be checked, which is not always the case.

The way the caller uses the result of this function is also wrong.
If the helper says "we do not want to let this push go through", the
caller unconditionally translates it into "we blocked it because the
destination already exists", which is not true at all in this case.

Fix this by doing these three things:

 * Remove unnecessary not_forwardable from "struct ref"; it is only
   used inside set_ref_status_for_push();

 * Make "refs/tags/" the only hierarchy that cannot be replaced
   without --force;

 * Remove the misguided attempt to force that everything that
   updates an existing ref has to be a commit outside "refs/tags/"
   hierarchy.

The policy last one tried to implement may later be resurrected and
extended to ensure fast-forwardness (defined as "not losing
objects", extending from the traditional "not losing commits from
the resulting history") when objects that are not commit are
involved (e.g. an annotated tag in hierarchies outside refs/tags),
but such a logic belongs to "is this a fast-forward?" check that is
done by ref_newer(); is_forwardable(), which is now removed, was not
the right place to do so.

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