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>