refactor: logger cmdSerializer

lib/logger/cmd-serializer.js

+module.exports = cmdSerializer;
+
+// istanbul ignore next
+function cmdSerializer(cmd) {
+  return cmd.replace(/https:\/\/[^@]*@/g, 'https://**redacted**@');
+}

lib/logger/index.js

@@ -3,15 +3,11 @@ const bunyan = require('bunyan');
 const PrettyStdout = require('./pretty-stdout').RenovateStream;
 const configSerializer = require('./config-serializer');
 const errSerializer = require('./err-serializer');
+const cmdSerializer = require('./cmd-serializer');
 
 let bunyanLogger;
 let meta = {};
 
-// istanbul ignore next
-function cmdSerializer(cmd) {
-  return cmd.replace(/https:\/\/[^@]*@/g, 'https://**redacted**@');
-}
-
 function initLogger() {
   const stdout = {
     name: 'stdout',