maintenance: respect remote.*.skipFetchAll

maintenance: respect remote.*.skipFetchAll

If a remote has the skipFetchAll setting enabled, then that remote is
not intended for frequent fetching. It makes sense to not fetch that
data during the 'prefetch' maintenance task. Skip that remote in the
iteration without error. The skip_default_update member is initialized
in remote.c:handle_config() as part of initializing the 'struct remote'.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>