Unless the "quiet" flag is given, check_connected sends any
errors to the stderr of the caller (because the child
rev-list inherits that descriptor). However, server-side
callers may want to send these over a sideband channel
instead. Let's make that possible.
Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>