bug#51130: 29.0.50; shell completion fails to complete filenames

[Stefan Monnier]

> commit e17d9003618cef75852a0b88334cea66a5c4b016
> Author:     Stefan Monnier <monnier@iro.umontreal.ca>
> AuthorDate: Sat Jun 4 09:31:34 2011 -0300
>
>     * lisp/shell.el: Require and use pcomplete.
>     (shell-dynamic-complete-functions): Add pcomplete-completions-at-point.
>     (shell-completion-vars): Set pcomplete-default-completion-function.
>
> Just leaving pcomplete-default-completion-function alone seems to give
> better results in shell-mode (after testing very briefly), so I'm not
> sure why that was added.

It was added because leaving `pcomplete-default-completion-function`
alone means that the functions after `pcomplete-completions-at-point` in
`shell-dynamic-complete-functions` are not used any more.

Maybe that's indeed what we want.

I don't use `shell` enough to make such a decision.
[ And I don't understand why there's both `shell-filename-completion`
  and `comint-filename-completion` in there either, which makes it even
  more clear to me that I don't know what such changes imply.  ]


        Stefan