Function std::pb_write_int64_field

void pb_write_int64_field(&String out, int field_num, long long v)

int32/int64: plain varint of the value's bit pattern (protobuf's own quirk: a negative int32 is sign-extended to 64 bits before varint encoding, so it always takes the full 10 bytes on the wire — this matches protoc's own behavior, not a bug).