Display the null SHA-1 as the base for an OBJ_OFS_DELTA.

Display the null SHA-1 as the base for an OBJ_OFS_DELTA.

Because we are currently cheating and never supplying the delta base
for an OBJ_OFS_DELTA we get a random SHA-1 in the delta base field.
Instead lets clear the hash out so its at least all 0's.  This is
somewhat more obvious that something fishy is going on, like we
don't actually have the SHA-1 of the base handy.  :)

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>