object: correct the expected ID size in prefix lookup

We take in a possibly partial ID by taking a length and working off of
that to figure out whether to just look up the object or ask the
backends for a prefix lookup.

Unfortunately we've been checking the size against `GIT_OID_HEXSZ` which
is the size of a *string* containing a full ID, whereas we need to check
against the size we can have when it's a 20-byte array.

Change the checks and comment to use `GIT_OID_RAWSZ` which is the
correct size of a git_oid to have when full.