ref_transaction_delete(): check that old_sha1 is not null_sha1
ref_transaction_delete(): check that old_sha1 is not null_sha1

It makes no sense to delete a reference that is already known not to
exist.

Signed-off-by: Michael Haggerty <c24c152f145c57791e809b126db9e61bd1c1782d@alum.mit.edu>
Reviewed-by: Stefan Beller <a7f3b1612c245dae606b3b9617b4f52187fddb65@google.com>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git