graft: don't preserve most extra fields This backs out changeset 804abe8999f3. See the previous patch for why we're doing this.

amend: don't preserve most extra fields This backs out changeset 39163708825cd. There are some extra fields that absolutely should not be preserved, like the convert_revision field introduced by the convert and hgsubversion extensions. The problem with extensions blacklisting certain extra fields is that they might not be enabled at the time the amend is performed. In the long run we probably want separately marked transferable and non-transferable extra fields, but for now restore the old Mercurial 3.6 behavior.