If we want to check whether an object is missing, the correct flag to
pass to rev-list is --ignore-missing; --exclude-promisor-objects will
exclude any object that came from the promisor remote, whether it is
present or missing. Use the correct flag.
Signed-off-by: Jonathan Tan <jonathantanmy@google.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>