ATtiny85 absolute ratings for outputs?
From the datasheet, you can see that all pins have protection diodes. This means that if you try to drive pins negative, they will start clamping voltage when the input goes below some voltage (which is guaranteed to be at least -0.5 volts).

You will drive input to -1 volt via resistor. In the worst case, the pin will be clamped at -0.5 volt, and you will be dropping 0.5 volt over 100 ohm resistor. This means pin current will be 5 milliamperes. Absolute 'DC Current per I/O pin' is 40 mA, and you are nowhere near this value. No need to worry.