index-pack --promisor: don't check blobs

index-pack --promisor: don't check blobs

As a follow-up to the parent of this commit, it was found that not
checking for the existence of blobs linked from trees sped up the fetch
from 24m47.815s to 2m2.127s. Teach Git to do that.

The tradeoff of not checking blobs is documented in a code comment.

(Blobs may also be linked from tag objects, but it is impossible to know
the type of an object linked from a tag object without looking it up in
the object database, so the code for that is untouched.)

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