bug#29007: 25.3; [PATCH] Make filecache use extended completion

[Andreas Politz <politza@hochschule-trier.de>]

OK, I'll elaborate.

Currently, file-cache-minibuffer-complete does longest-prefix completion
via all-completions only, instead of completion-all-completions, which
completes according to the completion-styles variable.  And here are
some cases where this behavior is inconvenient or even insufficient,
when trying to open some file via `C-x C-f C-TAB'.

1. Opening some file from the file-cache, without remembering the exact
   prefix.

   Example: You only remember that the file ends in '-foo.el'.

2. Opening some file, while the file-cache contains many files having the
   same prefix.

   Example: The cache contains the following files.
   
   aaa.a
   aab.b
   aba.c
   abb.d
   baa.e
   bab.f
   bba.g
   bbb.h

   Here prefix completion is basically useless, but entering the unique
   suffix and using substring completion immediately completes to the
   unique match.

-ap