Toggle file-name shadowing in minibuffers (File-Name Shadow mode).
With a prefix argument ARG, enable File-Name Shadow mode if ARG
is positive, and disable it otherwise. If called from Lisp,
enable the mode if ARG is omitted or nil.
File-Name Shadow mode is a global minor mode. When enabled, any
part of a filename being read in the minibuffer that would be
ignored (because the result is passed through
`substitute-in-file-name') is given the properties in
`file-name-shadow-properties', which can be used to make that
portion dim, invisible, or otherwise less visually noticeable.