CI check for uncommitted generated proto files (#3159)
CI check for uncommitted generated proto files (#3159)

Fixes #3087.

demo: 
![Unnamed screen capture (1)](https://user-images.githubusercontent.com/635073/88002082-82784480-cab6-11ea-8374-6461da3e91c5.gif)

quantumlib/Cirq,quantumlib/Cirq,quantumlib/Cirq,quantumlib/Cirq,balopat/Cirq,balopat/Cirq,quantumlib/Cirq,balopat/Cirq