The Listen gem does not notify across process forks, and so forked
processes must re-create their listeners.  `EventedFileUpdateChecker`
handled this in `updated?` by checking if `Process.pid` was different.
Because file updates may have occurred after the start of the fork but
before the call to `updated?`, `updated?` would always return true in
this case.

However, the above approach can result in unnecessary application
reloading, particularly when using Spring.  That, in turn, can cause
unexpected and problematic behavior: #39431, #37591.  This commit
changes listener re-creation to occur immediately after fork, and
changes `updated?` to return true only when necessary.