bug#21148: 25.0.50; Huge problems with case-insensitive search

[Sebastien Vauban]

As you can see on the screencast http://screencast.com/t/nq7xkfJiy,
case-insensitive [1] searches are becoming very fuzzy to me.

Sample text file:

--8<---------------cut here---------------start------------->8---
Let's search for all case-insensitive occurrences of "workingd"...

#+begin_src shell :var workingDir=(file-name-directory (buffer-file-name))
# No init file.
cmd="$EMACS -q -l ${workingDir}.init.el"
#+end_src
--8<---------------cut here---------------end--------------->8---

Recipe:

- Incremantally search (with `C-s') for "workingd"

Problems:

- While point is at the beginning of the buffer, and while the string
  "workingd" exists in the buffer, Isearch tells me from the start that
  it fails finding my pattern!??

- As the search is case-insensitive, `C-s' should allow me to cycle
  between the 3 strings found (1 "workingd" and 2 "workingD"), but it
  finally only finds the exact match "workingd".

Note -- Anzu correctly reports me (in the modeline) that there are
        3 found strings...

In GNU Emacs 25.0.50.1 (i686-pc-mingw32)
 of 2015-07-03 on LEG570
Repository revision: 2b848fadd51e805b2f46da64c5958ea7f009048a
Windowing system distributor `Microsoft Corp.', version 6.3.9600
Configured using:
 `configure --host=i686-pc-mingw32 --enable-checking=yes,glyphs'

Configured features:
XPM JPEG TIFF GIF PNG RSVG SOUND NOTIFY ACL GNUTLS LIBXML2 ZLIB
TOOLKIT_SCROLL_BARS

[1]  ┌────
     │ case-fold-search is a variable defined in ‘C source code’.
     │ Its value is t
     │ 
     │   Automatically becomes buffer-local when set.
     │ 
     │ Documentation:
     │ Non-nil if searches and matches should ignore case.
     └────