bug#9591: 24.0.50; buffer name completion

[Eli Zaretskii <eliz@gnu.org>]

> From: Stefan Monnier <monnier@iro.umontreal.ca>
> Date: Sun, 25 Sep 2011 21:18:30 -0400
> Cc: 9591@debbugs.gnu.org
> 
> > If I type C-x k mes TAB, it completes to ` *message-viewer RMAIL'.
> 
> Hmm... that's probably a bug of the `substring' completion-style

But `substring' is not in the default value of completion-styles.  So
how does it come into play here?

By the way, this documentation of the `emacs22' style:

  Prefix completion that only operates on the text before point.
  I.e. when completing "foo_bar" (where _ is the position of point),
  it will consider all completions candidates matching the glob
  pattern "foo*" and will add back "bar" to the end of it.

and a similar doc string of `basic', are either inaccurate/wrong (it
never tells explicitly whether the "glob pattern" is interpreted as
starting at the beginning of each candidate, but if it doesn't, why
talk about "prefix"?), or there's a bug in the implementation of these
styles, because I just customized completion-styles to include only
one style, either `emacs22' or `basic', and I still see that "C-x b
Mes TAB" completes to "*Messages*".  Did I do something wrong?

If these completion styles are _supposed_ to match the "glob pattern"
not only at the beginning, then we should have an additional style
that does.

I think that fixing this will go a long way towards resolving
Richard's problem, because he will then be able to customize
completion-styles and have what he wants: the traditional, less
"imaginative", but much more predictable style of completion.

P.S. FWIW, I have no problems with the default completion operation,
but that's me.