Mention that Phabricator users should subscribe to *-commits

This probably explains why a lot of messages get lost for first time
Phabricator users.
Add vfs::recursive_directory_iterator

For now, this is only used by its unit tests.  It is similar to the API
in llvm::sys::fs::recursive_directory_iterator, but without some of the
more complex features like requesting that the iterator not recurse into
the next directory, for example.