push: don't fetch commit object when checking existence

push: don't fetch commit object when checking existence

If we're checking to see whether to tell the user to do a fetch
before pushing there's no need for us to actually fetch the object
from the remote if the clone is partial.

Because the promisor doesn't do negotiation actually trying to do
the fetch of the new head can be very expensive as it will try and
include history that we already have and it just results in rejecting
the push with a different message, and in behavior that is different
to a clone that is not partial.

Signed-off-by: Tom Hughes <tom@compton.nu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>