When mounting using relative paths, "touch" on directories
used to fail.

"touch" on files works accidentially because it uses a
file descriptor internally.

  $ loopback b a &
  $ cd b
  $ mkdir foo
  $ touch foo
  touch: setting times of 'foo': Not a directory

strace:

  [pid 30185] utimensat(0, "a/foo", [{1468441847, 0}, {1468441847, 0}], AT_SYMLINK_NOFOLLOW <unfinished ...>
  [pid 30185] <... utimensat resumed> )   = -1 ENOTDIR (Not a directory)