check_connected: add progress flag

check_connected: add progress flag

Connectivity checks have to traverse the entire object graph
in the worst case (e.g., a full clone or a full push). For
large repositories like linux.git, this can take 30-60
seconds, during which time git may produce little or no
output.

Let's add the option of showing progress, which is taken
care of by rev-list.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>