clone: do faster object check for partial clones

clone: do faster object check for partial clones

For partial clones, doing a full connectivity check is wasteful; we skip
promisor objects (which, for a partial clone, is all known objects), and
enumerating them all to exclude them from the connectivity check can
take a significant amount of time on large repos.

At most, we want to make sure that we get the objects referred to by any
wanted refs. For partial clones, just check that these objects were
transferred.

Signed-off-by: Josh Steadmon <steadmon@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>