diff_aligned_abbrev: use "struct oid"

diff_aligned_abbrev: use "struct oid"

Since we're modifying this function anyway, it's a good time
to update it to the more modern "struct oid". We can also
drop some of the magic numbers in favor of GIT_SHA1_HEXSZ,
along with some descriptive comments.

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