t5318: update for SHA-256

Switch two tests to use $ZERO_OID to represent the all-zeros object ID.

Signed-off-by: brian m. carlson <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: bk2204

t/t5318-commit-graph.sh

@@ -629,7 +629,7 @@ test_expect_success 'corrupt commit-graph write (broken parent)' '
 		empty="$(git mktree </dev/null)" &&
 		cat >broken <<-EOF &&
 		tree $empty
-		parent 0000000000000000000000000000000000000000
+		parent $ZERO_OID
 		author whatever <[email protected]> 1234 -0000
 		committer whatever <[email protected]> 1234 -0000
 
@@ -650,7 +650,7 @@ test_expect_success 'corrupt commit-graph write (missing tree)' '
 		cd repo &&
 		tree="$(git mktree </dev/null)" &&
 		cat >broken <<-EOF &&
-		parent 0000000000000000000000000000000000000000
+		parent $ZERO_OID
 		author whatever <[email protected]> 1234 -0000
 		committer whatever <[email protected]> 1234 -0000