Has anyone programmed ATmega8A using USBasp?
Your setup might just be flawed.  

I have programmed not specifically the ATmega8, but I have done an ATmega16 which is the same process.  I note the article on my website which has been used by many.

At one point in the article I list a command: 

-c usbasp -p atmega16 -B12 -U flash:w:"$(ProjectDir)Debug\$(ItemFileName).hex":i


You will change this to

-c usbasp -p atmega8 -B12 -U flash:w:"$(ProjectDir)Debug\$(ItemFileName).hex":i