check_everything_connected(): refactor to use an iterator

We will be using the same "rev-list --verify-objects" logic to add a
sanity check to the receiving end of "git push" in the same way, but the
list of commits that are checked come from a structure with a different
shape over there.

Update the function to take an iterator to make it easier to reuse it in
different contexts.

Signed-off-by: Junio C Hamano <gitster@pobox.com>