kokora: checkout matching grpc/grpc branch for xds test (#3459)

test/kokoro/xds.sh

@@ -7,10 +7,16 @@ cd github
 
 export GOPATH="${HOME}/gopath"
 pushd grpc-go/interop/xds/client
+branch=$(git branch --all --no-color --contains "${KOKORO_GITHUB_COMMIT}" \
+    | grep -v HEAD | head -1)
+shopt -s extglob
+branch="${branch//[[:space:]]}"
+branch="${branch##remotes/origin/}"
+shopt -u extglob
 go build
 popd
 
-git clone https://github.com/grpc/grpc.git
+git clone -b "${branch}" https://github.com/grpc/grpc.git
 
 grpc/tools/run_tests/helper_scripts/prep_xds.sh
 GRPC_GO_LOG_VERBOSITY_LEVEL=99 GRPC_GO_LOG_SEVERITY_LEVEL=info \