Add testcase for needless objects during a shallow fetch

Add testcase for needless objects during a shallow fetch

This is a testcase that checks for a problem where, during a specific
shallow fetch where the client does not have any commits that are a
successor of the new shallow root (i.e., the fetch creates a new
detached piece of history), the server would simply send over _all_
objects, instead of taking into account the objects already present in
the client.

The actual problem was fixed by a recent patch series by Nguyễn Thái
Ngọc Duy already.

Signed-off-by: Matthijs Kooijman <matthijs@stdin.nl>
Signed-off-by: Junio C Hamano <gitster@pobox.com>