t0410: use from-scratch server

t0410: use from-scratch server

A subsequent commit will add functionality: when fetching from a
promisor remote, existing non-promisor objects that are ancestors of any
fetched object will be repacked into promisor packs (since if a promisor
remote has an object, it also has all its ancestors).

This means that sometimes, a fetch from a promisor remote results in 2
new promisor packs (instead of the 1 that you would expect). There is a
test that fetches a descendant of a local object from a promisor remote,
but also specifically tests that there is exactly 1 promisor pack as
a result of the fetch. This means that this test will fail when the
subsequent commit is added.

Since the ancestry of the fetched object is not the concern of this
test, make the fetched objects have no ancestry in common with the
objets in the client repo. This is done by making the server from
scratch, instead of using an existing repo that has objects in common
with the client.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>