repository: Fix passing zero OID to WriteRef
In ceb9161b (repository: Resolve revisions passed to the WriteRef RPC,
2022-07-29) we have refactored the WriteRef()
RPC to resolve both old
and new revision before executing git-update-ref(1) in order to ensure
that the objects we want to update to actually exist. This refactoring
broke the case though where the client passes the all-zeroes object ID
either as old or new reference, which both have a well-defined meaning
in this context.
Fix this bug by special-casing the all-zeroes object ID.