fetch-pack: add refetch

fetch-pack: add refetch

Allow a "refetch" where the contents of the local object store are
ignored and a full fetch is performed, not attempting to find or
negotiate common commits with the remote.

A key use case is to apply a new partial clone blob/tree filter and
refetch all the associated matching content, which would otherwise not
be transferred when the commit objects are already present locally.

Signed-off-by: Robert Coup <robert@coup.net.nz>
Signed-off-by: Junio C Hamano <gitster@pobox.com>