connected: always use partial clone optimization

With 50033772d5 ("connected: verify promisor-ness of partial clone",
2020-01-30), the fast path (checking promisor packs) in
check_connected() now passes a subset of the slow path (rev-list) - if
all objects to be checked are found in promisor packs, both the fast
path and the slow path will pass; otherwise, the fast path will
definitely not pass. This means that we can always attempt the fast path
whenever we need to do the slow path.

The fast path is currently guarded by a flag; therefore, remove that
flag. Also, make the fast path fallback to the slow path - if the fast
path fails, the failing OID and all remaining OIDs will be passed to
rev-list.

The main user-visible benefit is the performance of fetch from a partial
clone - specifically, the speedup of the connectivity check done before
the fetch. In particular, a no-op fetch into a partial clone on my
computer was sped up from 7 seconds to 0.01 seconds. This is a
complement to the work in 2df1aa239c ("fetch: forgo full
connectivity check if --filter", 2020-01-30), which is the child of the
aforementioned 50033772d5. In that commit, the connectivity check
*after* the fetch was sped up.

The addition of the fast path might cause performance reductions in
these cases:

 - If a partial clone or a fetch into a partial clone fails, Git will
   fruitlessly run rev-list (it is expected that everything fetched
   would go into promisor packs, so if that didn't happen, it is most
   likely that rev-list will fail too).

 - Any connectivity checks done by receive-pack, in the (in my opinion,
   unlikely) event that a partial clone serves receive-pack.

I think that these cases are rare enough, and the performance reduction
in this case minor enough (additional object DB access), that the
benefit of avoiding a flag outweighs these.

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

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