Fix default projectRoot + watchFolders computation

Summary:
@public

This diff fixes the `projectRoot` calculation on React Native when the app does not have a config file. This should fix the issues reported in facebook#20712

Reviewed By: hramos

Differential Revision: D9444982

fbshipit-source-id: 4cb41fa5224d2addf92976cc119e49dea6a6346b

Author: rafeca

local-cli/util/Config.js

@@ -21,7 +21,7 @@ const {loadConfig} = require('metro-config');
  */
 import type {ConfigT} from 'metro-config/src/configTypes.flow';
 
-function getProjectPath() {
+function getProjectRoot() {
   if (
     __dirname.match(/node_modules[\/\\]react-native[\/\\]local-cli[\/\\]util$/)
   ) {
@@ -44,12 +44,12 @@ const resolveSymlinksForRoots = roots =>
     [...roots],
   );
 
-const getProjectRoots = () => {
+const getWatchFolders = () => {
   const root = process.env.REACT_NATIVE_APP_ROOT;
   if (root) {
     return resolveSymlinksForRoots([path.resolve(root)]);
   }
-  return resolveSymlinksForRoots([getProjectPath()]);
+  return [];
 };
 
 const getBlacklistRE = () => {
@@ -76,17 +76,15 @@ const Config = {
       ],
       getPolyfills,
     },
-
-    watchFolders: [getProjectPath(), ...getProjectRoots()],
+    watchFolders: getWatchFolders(),
     transformModulePath: require.resolve('metro/src/reactNativeTransformer'),
   },
 
-  getProjectPath,
-  getProjectRoots,
-
   async load(configFile: ?string): Promise<ConfigT> {
+    const argv = {cwd: getProjectRoot()};
+
     return await loadConfig(
-      configFile ? {config: configFile} : {},
+      configFile ? {...argv, config: configFile} : argv,
       this.DEFAULT,
     );
   },