odb: avoid hashing twice in and edge case

If none of the backends support direct writes and we must stream the
whole file, we already know what the object's id should be; so use the
stream's functions directly, bypassing the frontend's hashing and
overwriting of our existing id.

src/odb.c

@@ -864,8 +864,8 @@ int git_odb_write(
 	if ((error = git_odb_open_wstream(&stream, db, len, type)) != 0)
 		return error;
 
-	git_odb_stream_write(stream, data, len);
-	error = git_odb_stream_finalize_write(oid, stream);
+	stream->write(stream, data, len);
+	error = stream->finalize_write(stream, oid);
 	git_odb_stream_free(stream);
 
 	return error;