t4202: abstract away SHA-1-specific constants

t4202: abstract away SHA-1-specific constants

Adjust the test so that it computes values for object IDs instead of
using hard-coded hashes.  Additionally, update the sanitize_output
function to sanitize the index lines in diff output, since it's clear
from the assertions in question that we are not interested in the
specific object IDs.

Signed-off-by: brian m. carlson <redacted>
Signed-off-by: Junio C Hamano <redacted>