bug#7215: 24.0.50; Minibuffer path completion mispositions point

[Tim Van Holder <tim.vanholder@gmail.com>]

Recipe:

$ mkdir directory
$ cd directory
$ mkdir subdir
$ cd subdir
$ emacs -Q
  C-x C-f

At this point, the minibuffer will contain (with * indicating point):

  Find file: .../directory/subdir/*

Now delete most of "directory", leaving the minibuffer as:

  Find file: .../dir*/subdir/

Now press [tab]. This correctly completes "dir" back to "directory", but
also moves point backwards, resulting in:

  Find file: .../di*rectory/subdir/

The amount point moves seems to be relative to the place it should end
up at (it will always be after the "di" regardless of whether the
completion was for "di", "dir" or "direc"). It also seems to be related
to the size of the subsequent path component; doing the same tab
completion for the portion of the path before "directory" makes point
jump much further back.

For the last path component (subdir in this case) it _looks_ fine,
resulting in

  Find file: .../directory/subdir*/

But simply adding a single letter after the trailing '/' shows that this
is just the one case where it looks like it's doing the right thing;
trying to complete "subdir" from "sub" then yields:

  Find file: .../directory/subdi*r/x


In GNU Emacs 24.0.50.2 (i686-pc-linux-gnu, GTK+ Version 2.8.20)
 of 2010-10-14 on leeloo
Windowing system distributor `The Cygwin/X Project', version 11.0.10503000
configured using `configure  '--with-x''