fsevents: fix subfolder check

First of all, a bit of explanation of what happens there:

1. FSEvents emits absolute paths to changed files or directories
2. We cut off the first part of such paths, which is equal to handle's
   real path ('/dir/subdir/subsubdir`, without trailing slash)
3. Then, if we are running in non-recursive mode, we discard paths that
   have slashes ('/') as a non-first character in them

/cc @bnoordhuis