sint64() {
        let [lo, hi] = this.varint64();
        // decode zig zag
        let s = -(lo & 1);
        lo = ((lo >>> 1 | (hi & 1) << 31) ^ s);
        hi = (hi >>> 1 ^ s);
        return new pb_long_1.PbLong(lo, hi);
    }
Read a `sint64` field, a signed, zig-zag-encoded 64-bit varint.