fetch: forgo full connectivity check if --filter

fetch: forgo full connectivity check if --filter

If a filter is specified, we do not need a full connectivity check on
the contents of the packfile we just fetched; we only need to check that
the objects referenced are promisor objects.

This significantly speeds up fetches into repositories that have many
promisor objects, because during the connectivity check, all promisor
objects are enumerated (to mark them UNINTERESTING), and that takes a
significant amount of time.

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