🐛 Fix bug disposing watchers in Project::addPath

`this.rootDirectories` is an Array of Directory objects. `path` is a
String. Therefore, `this.rootDirectories.includes(path)` will always
evaluate to `false`. We instead need to look for an entry in
`this.rootDirectories` where the Directory object's path is equal to the
given path.

src/project.js

@@ -347,7 +347,7 @@ class Project extends Model {
     })
 
     for (let path in this.watcherPromisesByPath) {
-      if (!this.rootDirectories.includes(path)) {
+      if (!this.rootDirectories.find(dir => dir.getPath() === path)) {
         this.watcherPromisesByPath[path].then(watcher => { watcher.dispose() })
       }
     }