Add .gitattributes file to hide proto generated code in PRs (#3065)
Add .gitattributes file to hide proto generated code in PRs (#3065)

This will cause generated code to be folded by default when viewing
diffs in PRs that touch protobuf definitions. We have automated checks
to ensure that the generated code is updated, so there's typically no
need to look at it.
balopat/Cirq,balopat/Cirq,quantumlib/Cirq,quantumlib/Cirq,quantumlib/Cirq,quantumlib/Cirq,balopat/Cirq,quantumlib/Cirq