nth_packed_object_oid(): use customary integer return

nth_packed_object_oid(): use customary integer return

Our nth_packed_object_sha1() function returns NULL for error. So when we
wrapped it with nth_packed_object_oid(), we kept the same semantics. But
it's a bit funny, because the caller actually passes in an out
parameter, and the pointer we return is just that same struct they
passed to us (or NULL).

It's not too terrible, but it does make the interface a little
non-idiomatic. Let's switch to our usual "0 for success, negative for
error" return value. Most callers either don't check it, or are
trivially converted. The one that requires the biggest change is
actually improved, as we can ditch an extra aliased pointer variable.

Since we are changing the interface in a subtle way that the compiler
wouldn't catch, let's also change the name to catch any topics in
flight. We can drop the 'o' and make it nth_packed_object_id(). That's
slightly shorter, but also less redundant since the 'o' stands for
"object" already.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>