sha1dc: disable safe_hash feature

sha1dc: disable safe_hash feature

The safe_hash feature is designed to make sha1dc a drop-in
replacement for sha1, where colliding entries will get a
permuted hash to un-collide them. However, since we're
handling the collision case ourselves, this isn't helpful
(and is actually harmful, as it means you get the wrong
object id if you want to show it in a log message).

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>