bug#42383: 28.0.50; Two bugs with M-x compile

[Gregory Heytings via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

>
> ** Bug 1 **
>
> Steps to reproduce:
>
> 1. create a Makefile with a few targets
> 2. start emacs -Q
> 3. M-x compile
> 4. press TAB: the list of targets is displayed
> 5. type the first character of an existing target, and press TAB
> 6. the result is a "[No match]"
>
> This has been working correctly for years (tested with Emacs 24, 25, 
> 26), and does not work anymore with Emacs 28.
>
> It seems that at step 5 above the list of completion candidates that is 
> considered are subdirectories.
>

Upon further investigation, this bug has been introduced by the patch for 
bug#34330, which moved pcomplete-completions-at-point last in 
shell-dynamic-complete-functions in shell.el:

(defcustom shell-dynamic-complete-functions
    '(comint-c-a-p-replace-by-expanded-history
      shell-environment-variable-completion
      shell-command-completion
      shell-c-a-p-replace-by-expanded-directory
      shell-filename-completion
      comint-filename-completion
      ;; Put `pcomplete-completions-at-point' last so that other
      ;; functions can run before it does, see bug#34330.
      pcomplete-completions-at-point)

See https://git.savannah.gnu.org/cgit/emacs.git/commit/?id=e4d17d8cb479ffeeb7dfb7320a1432722ac8df75 .

The obvious fix is to revert the situation (that is, to move 
pcomplete-completions-at-point again before shell-filename-completion), 
but I'm not sure, because bug#34330 would again exist.  That being said, 
I'm not sure that handling the exceptional case of filenames with '&' 
characters correctly when completing in M-x shell (which is what bug#34330 
is about) justifies to break normal behavior in other situations.

Gregory