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.