connected: verify promisor-ness of partial clone

connected: verify promisor-ness of partial clone

Commit dfa33a298d ("clone: do faster object check for partial clones",
2019-04-21) optimized the connectivity check done when cloning with
--filter to check only the existence of objects directly pointed to by
refs. But this is not sufficient: they also need to be promisor objects.
Make this check more robust by instead checking that these objects are
promisor objects, that is, they appear in a promisor pack.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>