hash: require hash algorithm in `is_empty_{blob,tree}_oid()`

hash: require hash algorithm in `is_empty_{blob,tree}_oid()`

Both functions `is_empty_{blob,tree}_oid()` use `the_repository` to
derive the hash function that shall be used. Require callers to pass in
the hash algorithm to get rid of this implicit dependency.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>