Dynamically expand and complete the filename at point.
Replace the filename with an expanded, canonicalized and
completed replacement, i.e. substituting environment
variables (e.g. $HOME), `~'s, `..', and `.', and making the
filename absolute. For expansion see `expand-file-name' and
`substitute-in-file-name'. For completion see
`comint-dynamic-complete-filename'.
(fn)