Summary:
This diff tries to fix an issue that started appearing in RN master when the latest metro version is used (more info here: c5ce762697 (commitcomment-29443117)).
The problem is that RN is still reading `projectRoots` from the Metro config and trying to call `concat()` on them. Latest Metro sets the default `projectRoots` to null, so this fails.
Also, a couple other things have been done:
* Make sure that the `projectRoot` and `watchFolder` objects from args are passed to Metro (so they contain the overrides from the CLI arguments).
* Add a `--projectRoot` CLI arg, replacing the `--root` one (this is not actually needed, since it does the same as `--watchFolders`
Differential Revision: D8567229
fbshipit-source-id: 3b76fd8e23d201a4097e9dfba3a512d64f348cb0