Update setPath to work with custom files

[hansonw]

Currently, setPath creates a new File object. If we already have a custom file object we should just call setPath on it, along with the other setup to update the subscriptions and send the event.

Contributed under CC0.

[hansonw]

@maxbrunsfeld could you take a look at this when you get a chance? much appreciated :)

[maxbrunsfeld]

Just so I understand - why is this preferable to just calling setFile with a new instance of the same file class? Up until now, I had envisioned setPath(filePath) as being a convenient shorthand for
setFile(new pathwatcher.File(filePath)). I'm open to changing that behavior though.

[hansonw]

Just convenience I guess - for example, creating remote files in Nuclide requires a fair amount of dependencies. It's a bit roundabout to have to create a new file when File implementations are able to change their own paths without much of an issue. The current behavior also means that we've effectively had to "blacklist" TextEditor.setPath from the Nuclide codebase, which seems a little odd.