t7064: use update-ref -d to remove upstream branch

The previous code tested this by writing $ZERO_OID explicitly in the packed-refs
file. This is a type of corruption that doesn't reflect realistic use-cases. In
addition, even the ref-store test-tool refuses to write invalid OIDs.
(update-ref interprets $ZERO_OID is deleting the ref).

Signed-off-by: Han-Wen Nienhuys <hanwen@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t7064-wtstatus-pv2.sh

@@ -373,10 +373,7 @@ test_expect_success 'verify upstream fields in branch header' '
 
 		## Test upstream-gone case. Fake this by pointing
 		## origin/initial-branch at a non-existing commit.
-		OLD=$(git rev-parse origin/initial-branch) &&
-		NEW=$ZERO_OID &&
-		mv .git/packed-refs .git/old-packed-refs &&
-		sed "s/$OLD/$NEW/g" <.git/old-packed-refs >.git/packed-refs &&
+		git update-ref -d refs/remotes/origin/initial-branch &&
 
 		HUF=$(git rev-parse HEAD) &&