What kind of "information" are you trying to get from this signal?
Since you seem to already know it's 5V (peak-to peak?) at 50Hz, I'm pretty sure you don't need the negative-half of the waveform. And, since you know it's 5V, the 0.5 to 0.7V drop across a diode shouldn't be an issue either.
If you're looking for the zero-crossing, a diode will mess that up slightly, but that's no big deal because the time between the zero crossing and any phase angle along the waveform is constant. You can calculate it, or write some experimental test-code to find the timing that works best. And, you don't need to sense both positive-going and negative-going zero-crossings, because once you find one zero crossing, you know the next one is coming 1/100th of a second later.