cli: don't check for updates to npm when we are updating npm itself (#32)

PR-URL: #32
Credit: @olore
Reviewed-By: @zkat

Author: olore

bin/npm-cli.js

@@ -69,12 +69,15 @@
     npm.command = 'help'
   }
 
+  var isGlobalNpmUpdate = conf.global && ['install', 'update'].includes(npm.command) && npm.argv.includes('npm')
+
   // now actually fire up npm and run the command.
   // this is how to use npm programmatically:
   conf._exit = true
   npm.load(conf, function (er) {
     if (er) return errorHandler(er)
     if (
+      !isGlobalNpmUpdate &&
       npm.config.get('update-notifier') &&
       !unsupported.checkVersion(process.version).unsupported
     ) {