Do not add parent directory to file system monitoring

`EventedFileUpdateChecker` will search the parent directory if the
specified directory does not exist.

Since `test/mailers/previews` is included in the watch target by default,
if there is no test directory (e.g. using `rspec`), the Rails root directory
will be included in the watch target.

```
$ rails new app
$ cd app
$ ./bin/rails r "p Rails.application.reloaders.last.send(:directories_to_watch).include?(Rails.root)"
false
$ rm -rf test
$ ./bin/rails r "p Rails.application.reloaders.last.send(:directories_to_watch).include?(Rails.root)"
true
```

This causes `node_modules` to be included in watch target. Adding parent
directories to watch target may include unexpected directories.

In order to avoid this, fixed that parents of nonexistent directories are
not added to the watch targets, instead checking that the directory
exists when checking changes.

Related to #32700.

[Matthew Draper & Yuji Yaginuma]

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