diff: pass whole pending entry in blobinfo

diff: pass whole pending entry in blobinfo

When diffing blobs directly, git-diff picks the blobs out of
the rev_info's pending array and copies the relevant bits to
a custom "struct blobinfo". But the pending array entry
already has all of this information (and more, which we'll
use in future patches). Let's just pass the original entry
instead.

In practice, these two blobs are probably adjacent in the
revs->pending array, and we could just pass the whole array.
But the current code is careful to pick each blob out
separately and put it into another array, so we'll continue
to do so and make our own array-of-pointers.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>